ONE OF THE first things Susan Sheehan learned to do while researching her latest book was to keep her back to the wall. Mental patients have a tendency to hit when your back is turned.

One man threw a chair at her. He missed. Others would just give her a whack as they passed by, unexpected, unprovoked.

She visited Creedmoor Psychiatric Center in Queens, N.Y., over a period of two years, sometimes spending the day, other times sleeping there or staying the weekend. The product was "Is There No Place on Earth for Me?"--a book that captures in extraordinary detail and measured, unsentimental prose the life of a woman she calls Sylvia Frumkin, who is a schizophrenic. It appeared first as a series in The New Yorker and provoked an unusual 500 letters of response from readers.

During the research for her book, Sheehan commuted between her orderly and serene home off Foxhall Road NW and the mental institution, a place where "the floor became sticky with spilled soda and coffee, and powdery with cigarette ashes, 10 minutes after the latest wet mopping. The ward smelled of coffee, stale cigarette smoke and unwashed and incontinent patients," she wrote.

When she first presented the idea of writing about a mental institution to William Shawn, her editor at The New Yorker, he said it was too hard. "He was right," she said recently. Doing the book was "emotionally devastating." She says that matter of factly, as though it were merely an occupational hazard, like writer's cramp. There were days when Frumkin called her eight times from a pay phone in the hospital lobby. After the four-part series appeared, whenever she called Sylvia (she believes she should not "abandon" her subjects), Frumkin's mother would berate her for including her age and hair color, even though the name and neighborhood of the family had been changed to protect their identity. Last fall Sheehan found she had a bleeding ulcer.

"For the first time, Neil [Sheehan's husband] called Mrs. Frumkin and asked her to lay off . . . I'm not saying aggravation causes ulcers, but it was a great relief not to have her lighting into me again and again."

She continues to see Frumkin regularly, for lunch or a movie. "She's my interpreter of popular culture," she said. "She reads all the reviews and always knows what everything's about. For example, when we saw the movie 'Airplane,' I didn't get it because I hadn't seen the movies it was spoofing. Sylvia explained it all to me."

Frumkin liked the book, Sheehan said. Since the publication of the four articles in The New Yorker in the spring of last year, Frumkin has been seeing psychiatrist Gideon Seaman, the book's unnamed expert in psychopharmacology, and has lost some of the 200 pounds she had accumulated by the summer of 1980.

Frumkin is capable of extraordinary monologues, delivered at "Indianapolis 500 speed," that mix references to television, movies and literature with insights that are sometimes piercing and at other times just funny. But if there is one thing that comes through clearly in Shee-See SHEEHAN, G6, Col. 1 SHEEHAN, From G1 han's book, it is that insanity can be not simply a transitory spell of exploding emotions but a more or less permanent residence in a skewed and crippling world.

"I'm a doctor, you know. I don't have a diploma, but I'm a doctor. I'm glad to be a mental patient, because it taught me how to be humble. I use Cover Girl creamy natural make-up. Oral Roberts has been here to visit me. My sister's name is Joyce Frumkin, and I like her. My father is five feet two inches, my mother is five feet three inches. They're like Napoleon and Josephine, and they're shrinking. Joyce is five-two. I'm only five foot four and I'm the tallest one in my family. This is the place where Mad magazine is published. The Nixons make Noxon metal polish. When I was a little girl, I used to sit and tell stories to myself. When I was older, I turned off the sound on the TV set and made up dialogue to go with the shows I watched. The people in Creedmoor are Hobbits. I dictated the Hobbit stories to Tolkien and he took them all down. I'm the Hobbit. Ask John Denver. He told me I was . . . I have schizophrenia--cancer of the nerves. My body is overcrowded with nerves. This is going to win me the Nobel Prize for medicine."

Cancer of the nerves. Sylvia Frumkin is the lunatic poet of the nut house, summoning from the dark mess of her mind amazing screeds that ring like a bell in the fog. "The body is run by electricity. My wiring is all faulty . . ." she said once. On another day her vision was this: "My skin is just like the lawn. I'm going to tear it off and pluck out the bed of dandelions. This isn't schizophrenia, it's terminal acne . . ."

"Is There No Place on Earth For Me?" is a reportorial tour de force. Sheehan's technique, refined over the course of three major projects, is a kind of total immersion, observation and interviews combined with extensive library research. At Creedmoor, she insinuated herself so completely that she became the proverbial fly on the wall. She calls it "third person invisible."

Sheehan spent two months at Creedmoor before she found Sylvia Frumkin. She was looking for a schizophrenic because schizophrenia is the most common form of mental illness and the least treatable. "Some of the patients were too paranoid, some were too fragile," she said. "Others were too dangerous--the story might be good, but you might not live to tell it. It was Mrs. Plotnick the hospital unit chief who suggested Sylvia. She knew that she was articulate, and that she loved attention."

The book opens with Frumkin's eighth admittance to Creedmoor, in the midst of a psychotic breakdown. Sheehan was there. " 'There's your subject,' Mrs. Plotnick said, and Sylvia was delighted." They liked each other right away, she said, although the relationship subsequently endured the full range of Sylvia's psychotic behavior.

"At times she would turn on me," Sheehan said. "If you would warn her that she might be hassled by not taking her medication, she might not speak to you for a day. When she was in a born-again phase, she would try to convert me and wouldn't be satisfied with the answer that I was raised an Episcopalian and had been confirmed at 12. I knew it was no use arguing with her. You can't debate Sylvia Frumkin about Christianity."

Born in 1948, Frumkin saw her first therapist when she was 14; she had just turned 16 when she was initially admitted to Creedmoor. Over the 17 years of her life that Sheehan researched, she was in and out of Creedmoor 10 times and spent time in other hospitals as well. The longest period of time she was out of a hospital was for 20 months, between November 1970 and July 1972; the day before she was to graduate from a medical secretarial school she had another psychotic breakdown, although she got an "A" in the final project she had to complete for the course.

Eventually Sheehan accumulated more than 1,000 pages of notes (she takes them in speed writing) from her interviews and an equal number of pages from medical records and books. She interviewed every doctor in Frumkin's past, every member of her family, even her sister's therapist. She read more than 200 books, from R.D. Laing to "One Flew Over the Cuckoo's Nest," psychiatric textbooks and diagnostic manuals. She even re-read "Jane Eyre" to see how Charlotte Bronte described Rochester's mad wife.

She spent many evenings consulting Dr. Seamon, a psychiatrist and expert in psychopharmacology on the Creedmoor staff whom she refers to anonymously in the book but acknowledges in the forward. They worked from a 45-page, single-spaced abstract about Frumkin that Sheehan prepared, going through each series of drugs she had been given.

She relies on facts, dispassionately piled up like bricks, but equally on the knowledge that the sum of facts is not always the truth.The accumulation of truthful detail was the product of her labors, a job of reporting that placed her within the boundaries of intimacy yet outside the sphere of authority. At the times when she had to choose between intervening and observing, she invariably chose the reporter's role.

"You have to tell the story, not change it," she said. "Psychiatrists can play God. Writers can't."

If she had believed Frumkin to be suicidal, she would have intervened, Sheehan said, but on the occasions when Frumkin ran away from the hospital or a treatment center, she did not try to stop her.

Nor is she an adversarial investigative reporter. "I don't see heroes and villains, or black and white," she said. "I see gray." Even as a fly on the wall, she always identified herself as a writer. She carried a notebook openly, and used a tape recorder--for legal protection rather than notetaking. "The patients loved it. They wanted to sing into it." She never tried to camouflage her presence by dressing in anything other than her usual rather proper garb. Frumkin signed consent forms every six months, to assure Sheehan that she understood what she had agreed to.

Once when Sheehan was sleeping on a cot in Frumkin's room in the "hotel" unit at Creedmoor, the subject of her book took off: "The snow fell softly and steadily all through the night. Miss Frumkin put her nightgown on that evening, but she didn't go to sleep. She stayed up all night reading the Bible, rustling the pages of the New Testament noisily. For the first time in months, Miss Frumkin heard voices. The voices told her to leave the hospital immediately."

At that point, Sheehan had no idea what would happen to her subject, or whether she would even have a subject, and until Sylvia was found later in upstate New York, she worried that her entire project was in jeopardy. "It would have meant months of work down the drain."

Since publication of the book, Frumkin has not been hospitalized. She is planning to spend two weeks in a Weight Watchers camp, a gift from her sister Joyce. But she still lives at home, unhappily, and has continued her sporadic involvement with born-again Christians, a relationship Sheehan does not find encouraging.

The relationship of Sylvia and her aging parents--they were both 40 when she was born and are now in their seventies--has not improved. "I don't want to sound tasteless," said Sheehan, "but it will be interesting to see what happens to Sylvia when they die . . . I sometimes wish Mrs. Frumkin would get laryngitis for three months."

Sheehan is not optimistic about Sylvia's long-range prognosis, her ability to get a job or to live by herself. "She grates on people. I've gone with her on job interviews, and she'd invariably say something that would make them not want to hire her. And even before the SSI public assistance payments were cut, she could barely afford a studio apartment in New York. And you wonder if she could manage one on her own. If she fails, I worry that she might become psychotic again."

During one of her lucid periods, Frumkin reflected on her manic phase: "I'll miss having those fantasies. There's a charm to being sick. I like to be in the twilight zone of the real world. Absolutely real is getting up every day and going to work. I once thought . . . that I'd graduate and get a job. I was looking forward to earning my own money, to having a credit card, to being a grown woman in my own right . . . But if you can't have any of those things . . . When you know all those things exist for other people but not for you, sometimes it's very hard to endure the not having."

Sheehan's previous in-depth subjects, a welfare mother and a jailed burglar, were, like Frumkin, residents of the under class. Sheehan herself is a child of the upper middle class, a doctor's daughter who grew up on New York's Upper East Side, a Wellesley ('58) alumna married to a Harvard graduate and former New York Times reporter. She lives in the kind of neighborhood that is beyond the imagining of welfare mothers or public hospital mental patients (if not burglars), surrounded by light and beauty, two daughters and a live-in housekeeper.

Born in Vienna, she moved to New York at the age of 4. Her father died shortly afterwards, and her mother remarried a prominent obstetrician. After college she worked as an editorial assistant for Esquire and Coronet magazines, and then landed a job at The New Yorker. She left a "glamorous single life" to go to Djakarta with Neil Sheehan, then a reporter for The Times. They were married there. She got migraine headaches and didn't like the tropics. Her first book, "Ten Vietnamese," was produced two years later, published two months after her first daughter was born in 1967.

She feels at odds with her luxurious neighborhood, she said, with the joggers and tennis players and carpoolers. Because of an eyesight problem, she cannot drive a car. "I'm no use to them," she said, gesturing toward the street lined with expensive homes. Furthermore, she and her husband Neil generally write at night, going to bed at dawn after the morning paper has hit the doorstep. While most of their neighbors are off at law offices or government agencies, the Sheehans sleep.

For the last 16 years they have lived in Washington. She spends every third week in New York, staying at her mother's apartment on East 79th Street, and keeps two address books, one for each city. For 13 years she has had a study desk at the Library of Congress (a privilege granted only to serious users of the library), where she spends afternoons, a pattern she began when her children were little and she needed a place to work.

"I really want another life other than the one I have in Wesley Heights," she said. "Not being able to drive a car, I feel so imprisoned here . . . I really had to create another life for myself."

She took her two daughters, now 15 and 12, to Creedmoor, the mental hospital where Sylvia Frumkin was treated, and to the Brooklyn slums where the welfare mother lived. They spent one Christmas day with George Malinow, the convicted burglar she chronicled in "A Prison and a Prisoner." She took them partly because she wants them to share her life and partly as an antidote to the world of private schools and privilege that surrounds them.

"You want them to see people that are unfortunate, not just those who own BMWs and Porsches and belong to racist country clubs, people who think it's important to have an alligator on their shirt one year and a polo player the next . . . " she said. "I'm so chagrined that they go to a school National Cathedral School for Girls where there is a program to spend spring vacation in Italy or France; there are kids there who are driven to school in Rolls Royces. I don't want them to grow up thinking that's normal." Public school proved to be academically inadequate, she said somewhat apologetically, and she prizes a rigorous education. "When I was 15 I could speak four languages, I could go anywhere in South America or Europe and manage. I want that kind of independence and curiosity for them, and that they get a sense of how lucky they are."

Even a children's birthday expedition to the Source Theater turned into a social lesson, as the theater is on 14th Street and the return journey provided a panorama of downtown hustlers at work.

Her editorial home is The New Yorker, a magazine flush with advertisements distinctly aimed at the owners of BMWs and wearers of alligator shirts. For Sheehan, it has been a place where she has been allowed to blossom, starting as a writer of humor and the ruminative "Talk of the Town," moving on to the lengthy investigative pieces on which she now concentrates. Her kind of writing is luxurious journalism: free of time constraints and editorial interference, she can eschew newspaper shorthand, include at length details and anecdotes, and change names to permit even fuller honesty. She disputes the idea that The New Yorker is read largely by the upper middle class, pointing to the letters she has received in response to her pieces as evidence. "They are not from wealthy addresses, or on stationery that rich people would use," she says.

"Where else could you say to the editor, 'I want to write about an insane asylum,' and he says, 'fine,' and you come back a year and a half later with over 100,000 words, and he prints them," she said.

The spare, unadorned style she favors is quite conscious. "I learned that plainness can work as well as fanciness," she said. "Like the first line of 'A Welfare Mother': 'Carmen Santana is a welfare mother.' It looks so easy but it takes me two years to get there."She often rewrites a draft as many as 14 times. She can recite Yeats for half an hour, having memorized his work during a time when she was examining her own style. She rereads "Tender Is the Night" every year; her other stylistic models are the nonfiction works of E.B. White and Joseph Mitchell.

In 1978 she published a first-person piece in The New Yorker about an automobile accident her husband had in 1974. The accident put him in the hospital for two months and curtailed his earning abilities for almost a year; even today he takes medication for pain. It was a traumatic experience for them, and one of her ways of dealing with it was to write about the accident, the treatment and the perpetrator in dispassionate detail. The result revealed her reporter's reactions, as in this paragraph: "After Dr. Mourad told me that Neil had broken several ribs, I realized I didn't know how many ribs human beings have. I consulted a dictionary, learned that the number was twenty-four, and took what comfort I could from the fact that seventeen of Neil's were intact."

In reportorial fashion, they tracked down the man who had run into Neil Sheehan: He had an unregistered car, no insurance and a history of driving on the wrong side of the road.

She has spent the past year, after the page proofs of the book were finished, working on a new project she won't describe for fear of being scooped. And for the past few years, she also has been working intermittently on a biography of publisher Alfred Knopf, a project for which she was awarded a Woodow Wilson Fellowship in 1981. The Frumkin project was partly funded by a grant from the Ford Foundation because The New Yorker pays only once an article is ready for printing. To earn money for her children's tuition, she also does consulting work for the New York City Department of City Planning on the 42nd Street redevelopment project, which has the added benefit of being in the town she loves best.

The appeal of these subjects--Puerto Ricans on welfare, burglars in jail, a schizophrenic in a mental institution--is not the product of a clearly defined goal. Although Sheehan possesses a well-developed sense of indignation--remembering with outrage a man she believes kept her husband from winning a journalism prize, furious that her publishing company failed to get books distributed to cities she has visited for publicity, piqued at the "trust funders" who live next door inviting noisy guests to use their swimming pool all day--she is not a woman with a mission. She is a journalist with a sense of curiosity, a taste for the occasional adventure and a good story, and a need to do something that is neither frivolous nor too esoteric. After all, she pointed out, crime, welfare and craziness are subjects that are in the newspapers every day; she just wants to find out about them.

"I'm interested in people who are different from myself . . ." she said "Only serious subjects are worth the little time one has in life to write."

Sheehan traces her interest in mental illness to fragments of childhood memories, like the schizophrenic child of her mother's friend. "She would say things like, 'May I order the most expensive thing on the menu?'--which I knew was not correct behavior, although nobody every told me there was anything wrong with her." She also remembers being a guinea pig in a psychology class her mother took, brought in as an example of a 'normal' child to be tested in comparison to a troubled one. "This other kid saw murder and death in every inkblot," she remembers. "All I saw was happy families and fairy tales."

She hopes that the book will be useful to relatives of mental patients, to help them raise appropriate questions about hospital care and to warn them against specific therapies such as megavitamins and insulin shock. She was appalled at the general incompetence of both professional and support personnel, particularly, as the book records, at the cavalier prescription of drugs. "No treatment she (Frumkin) received over a period of 13 years bore any logical relationship to a previous treatment," said psychopharmacologist Seamon.

Sheehan does not like to write about something she can't find out everything there is to know about. She has to know about the black market in meat among the employes of Creedmoor as well as the history of schizophrenia. This kind of reporting is probably incompatible with politics, where secrecy and news management prevail, and celebrities, she says, are generally not worth the time.

"Is There No Place on Earth For Me?" has been more commercially successful than her others; the first printing of 14,000 has sold out, and a second of 5,000 has been ordered. The reviews have been raves, and the requests for interviews and speaking engagements numerous. There were two tentative movie offers, one from Lorimar Productions and another from Gilda Radner; although both came to naught, there have been other feelers.

For this success Sheehan also raises a glass to Sylvia Frumkin. "She deserves enormous consideration for having shared her life in that way. It was brave, and it was difficult."

"For a long time, I wanted to go on 'What's My Line?' as a mental patient. Being a mental patient really is my profession . . . But the show has gone off the air, and I may not be mentally ill anymore. Maybe I just have idiosyncracies--you'd have to ask a doctor . . . I want to be a doctor. I want to go from the theater to the operating theater. I'm going to be a doctor or a nurse, and I'm going to have the key to my front door . . . I'm a good friend of Mike Nichols and Diane von Furstenberg. I first met Geraldo Rivera when I was in Elmhurst. John Travolta's father is the Shadow. I think the Cowardly Lion was secrtely married to Judy Garland. I'm going to marry Lyle Waggoner, who plays Steve Trevor. I'm going to take Lynda Carter's place on "Wonder Woman" when I marry Steve. I want to have my own show, a show called 'Sylvia's.' I'm my favorite person. I only wish I could get along with everyone else as well as I get along with me. I secretly have my own show already."