IF YOU weren't supposed to stare, Diane Arbus -- starting with the flashers she forced herself to look at on the subway between her Park Avenue home and the fashionable Fieldston School in New York.

Later she watched her dying father with shocking objectivity, and photographed him dead. There was even a rumor that she had rigged a camera to photograph her own suicide in 1971 at age 48.

"It's possible," says Patricia Bosworth, author of "Diane Arbus," the first biography of the artist. "It's possible."

Though widely praised, exhibited, published -- and imitated -- for her stark snapshot-like images of nudists, transvestites, retardates, freaks and other forbidden subjects, Arbus has also been perceived as something of a freak herself, a voyeur.

"She was not as freaky and weird as people assumed just because of her fascination with freaks," insists Bosworth, a former model, actress and veteran New York writer and editor who spent 5 1/2 years trying to bring to life the story of this gifted photographer, born to wealth and privilege as the daughter of the owners of Russeks Fifth Avenue department store and flattened into mesmerizing myth since her death.

"Diane was a working woman, a wife and a mother. I wanted to try and paint a complete picture of her. I do not think she was a weirdo, nor do I think she was suicidal all her life. I think she was an original, and saw life in a way others do not. She could certainly see more in a face than I could."

And what faces! The pimply young man in a straw hat wearing a "Bomb Hanoi" button at a pro-Vietnam war rally. The grimacing, exasperated boy carrying a toy hand grenade. The like, yet hauntingly unlike, "Identical Twins." In fact, most of her best-known images had nothing to do with aberrations, and Arbus had good reason for her anger and frustration at being pegged "the photographer of freaks."

"But I think she was a voyeur, and I think I am too," admits Bosworth, whose early education in San Francisco's Convent of the Sacred Heart school did not inhibit her from relating startlingly intimate scenes from Arbus' life. "A biography is a voyeuristic act, and photography is too," she says. "I'm not proud of that, but I can't help myself. I think everybody is voyeuristic, really."

If proof be needed, publisher Alfred A. Knopf has moved swiftly into a second printing of the book, Avon has purchased paperback rights and MGM has bought the option to make a film. The Aperture book of Arbus photographs, published after her 1972 Museum of Modern Art retrospective, has sold a record 180,000 copies. Another Aperture book, on her magazine work, is due out this fall.

"The story is so unbelievably strong and dark and gothic in and of itself, that I felt if I superimposed judgments it would be too much," says Bosworth, who dutifully stuck to the facts, gleaned largely from 200 interviews with Arbus' relatives, friends, lovers and acquaintances. "Even Diane's brother, Howard Nemerov the Pulitzer Prize-winning poet , when I asked him about Diane's death, said, 'Don't be an armchair shrink, just tell the story.' "

And that she did, from the student days of the gifted golden girl called "Dee-yann," already obsessed in her youth with the outer limits of experience, to her last dark years, when despite what most artists would call success, she lost herself in a hunt for sensational sex, became ill with hepatitis and subsequently fell into a depression that neither doctor nor psychiatrist could stop. The results have astonished and dismayed her mother, sister and brother, even though all agreed to be interviewed.

None of them knew until they read the book, they say, of Diane's sexual "adventuring," as she called it, during the final years of her life among the '60s demimonde of New York's art and fashion world. None knew, they say, of the depth of the depression preceding her fail-safe suicide by barbiturates. (She also slashed her wrists.)

"Naturally, I was disturbed and depressed at the realization that we didn't know how unhappy she was, and wish we could have done more," said her younger sister Renee, an artist and technical illustrator now living in Albuquerque.

"I didn't know," said Howard Nemerov, whose more talkative wife Peggy said she read of Arbus' final years "with absolute horror. I thought, 'Why didn't you tell me, maybe I could have done something.' But after reading it, I felt there was nothing anybody could have done."

"I did nothing but cry," said Arbus' mother, Gertrude Nemerov, now 83 and also living in Albuquerque. "I had to go first through her death, and then this. This was worse."

Bosworth first met the highly successful fashion photography team of Diane and Allan Arbus in the 1950s, when she was an 18-year-old Powers model in New York. Allan Arbus -- now better known as the actor who played Dr. Sidney Freedman, the psychiatrist on "M*A*S*H" -- had been working in the advertising department at Russeks when he met Diane, then 14 years old. They married when she was 18, and divorced after 30 years, during which he taught his wife to use a camera.

"I went to their studio for a shoot," Bosworth says, "and they were wonderful -- very civilized, cultivated people, warm and friendly. The atmosphere in the studio was marvelous. I talked to many models who worked with them at the period, and they all loved to work for the 'Arbs,' as they were called, because they were so nice.

"I saw Diane occasionally in the '60s, when I was writing and editing various magazines," says Bosworth. "We were never close, but I would see her on the street, and we had coffee a couple of times. I remember going up the escalator with her at Lord & Taylor one day when she talked about her photography and complimented me on a piece I'd done. No more than that. The last time I saw her was at an Abbie Hoffman fundraiser, where she was photographing. She wore her camera everywhere. It was her shield."

Bosworth, the managing editor of Harper's Bazaar at the time, recalls the special milieu in which Arbus blossomed as an artist. "There was a lot of experimentation going on in all the magazines in the '60s, a lot of movement and action. Graphically, magazines were becoming more energizing visually . . . People were being photographed in odd and unusual ways, not only by Arbus. I remember Hiro photographing Faye Dunaway, and she hadn't shaved under her arms!

"Photographers were experimenting and doing all sorts of marvelous new kinds of portraiture. Arbus was at the forefront in the way she photographed, with black borders that looked like snapshots -- Avedon picked that up too. It was just a very exciting, innovative time, when photographers were becoming much more personal than they'd been before. They were also becoming part of the art world.

"It was my husband novelist Mel Arrighi who first suggested I write about Arbus," says Bosworth. He had also suggested her previous, best-selling biography, "Montgomery Clift." Was there some special fascination with suicides? "Clift was not technically a suicide -- he died of a heart attack," she says, adding stiffly, "I have no particular interest in the artist as suicidal."

Only when questioned later would Bosworth reveal that her own brother, Bartley Crum Jr., had died at age 19, an apparent suicide. "For me it is a very private tragedy," she said, and, "No one ever asked me.

"It's an excuse, perhaps. I said I was writing about a life, as opposed to a death, in both these books. But I suppose in the back of my mind I was also trying to figure out why a person would commit suicide. I still can't.

"I didn't offer the information, but I certainly don't deny it. I'd pushed it far into my subconscious. It's terribly traumatic for the survivor, you know."

Indeed, some of Arbus' survivors refused to cooperate with Bosworth, notably Diane's divorced husband Allan and their two daughters. Doon, 39, a writer, works for Richard Avedon. She invented the "Nothing comes between me and my Calvins" advertising campaign. Amy, 30, a photographer, shoots the "On the Street" column for the Village Voice. As executors of the estate, they also withheld permission to use any Arbus photographs in the book.

"They're coming out with their own Aperture book on Arbus' magazine work in October," Bosworth says. "It was probably a purely business decision."

"I have read the book," says Allan Arbus, who would not comment further, "but Doon and Amy have not." According to their grandmother, they made a pact last summer not to.

The only other major holdout was Arbus' closest male friend in her final years, Harper's Bazaar art director Marvin Israel, who found her body, and who took the secret of her last years to his own grave two weeks before publication of this book. "She was obsessed with him, utterly dependent upon him after Allan left for California," says Bosworth, who recounts a pathetic scene of Arbus, not long before her death, standing across the street from his house in the shadows, waiting for a glimpse of Israel or his wife.

Bosworth called Israel "brilliant, intimidating." (Peggy Nemerov referred to him as "creepy.") "He talked to me very early on and told me he would not cooperate with me. He told me the only person he wanted to do a book on Arbus was John Szarkowski from the Modern Museum, that he wanted her work explored.

"I remember him saying to me that he assumed I knew nothing about photography, and that I would not be able to understand Arbus. Strangely enough, because he said that, it made me work even harder."

Arbus would be 61 if she'd lived -- hard to believe, because she always looked so young. She was sometimes described as beautiful, at other times as gamy. People always spoke of her saucer-huge green eyes, her ability to make you think you were the only person in the world.

"She was a very sexual person, a very dependent person," says Bosworth. Others described her as a waif, "needy," lonely. "She smelled," said one blunt acquaintance.

"I think she was a chameleon in some ways," Bosworth says. "There were some days when she looked very chic, as she did to Walker Evans when she'd just come from seeing a Balenciaga show and was dressed in Courre ges. And then there were other times when somebody would notice her brown teeth and notice that she was a little disheveled.

"I think it had to do with her mood, but it also depended on the world she was going into. I think she wanted to be at her best for the Walker Evanses, whom she respected and revered, and perhaps when she took photographs of a transvestite in a welfare hotel, she might have looked a little bit disreputable herself so the transvestite would feel at ease."

And everyone spoke of her brilliance. "Arbus had begun to achieve mythic status before her suicide," Bosworth says. She was one of three photographers chosen by Museum of Modern Art photography curator John Szarkowski for his 1967 "New Documents" show (with Garry Winogrand and Lee Friedlander), which was heralded as the beginning of a new age in documentary photography. She had received two Guggenheim grants, and had been invited to show at the next Venice Biennale -- the first American photographer ever so honored.

But Walker Evans had asked her to teach at Yale, and she said no. She was asked to publish a book of her photographs, and said no. She was asked to do a one-person show at the Minneapolis Institute of Arts, and said no. Ultimately, in a profound depression, she said no to life.

"I don't know why she killed herself," says Bosworth.

"I do, of course, give perhaps superficial reasons. The fact that she was depressed and allergic to antidepressant drugs. That she'd had two bouts with hepatitis, which weakened her. You have to be very strong to be a photographer and wander around the city carrying all that equipment, making people pose for hours. The fact that she was weak upset and depressed her very much.

"Also the fact that Allan got married again -- she depended on him. That wasn't the only reason, but it was one of them. She was alone, and lonely, which can cause you to be very depressed. And her work was no longer giving her back what she wanted it to. It was the classic case of the artist who couldn't handle success.

"I think, on the one hand, she was very ambitious, but on the other hand, she had been raised to think a woman's place was in the home, that she should be a wife and a mother, and not have any ambition. I think she was very conflicted about it.

"Had she grown up later," Bosworth says, "her life might have been easier for her. I think she was sort of a free spirit, and there wouldn't have been the same kind of pressures on her, perhaps, because women were finding out that they had alternatives and choices.

"I think her suicide probably did turn her into a myth, or helped solidify her as a kind of legendary creature. Because suicide in and of itself is a mysterious, private act, I think artists who are connected with it tend to be romanticized -- like Sylvia Plath and Marilyn Monroe. She has been linked to them as a victim of society, but I don't think she was."

Could it be that Arbus played a role in creating her own myth? "Marvin Israel said she did. And in the last years, she talked obsessively about her 'adventuring,' as she called it. She had this need to tell stories, and she liked to shock and startle people. Marvin Israel called her a 'naughty little girl' for telling such tales, and she was.

"But she was very curious about nature, human nature and physical beings. I think her body and bodily functions were very interesting to her. This is something women now are able to talk about more freely -- their menstruation, or the pains of childbirth, or what have you. But in those days -- the 1940s -- it was unusual to be so interested, and to enjoy. She enjoyed her own body -- not in the end, perhaps. But as a young woman she enjoyed it, and was curious.

"I didn't think of her as a suicide, but as this fascinating creature I wanted to explore," Bosworth says, "the opposite of what people imagined her to be about. For instance, people say she was so unhappy, and yes, she was unhappy at the end of her life. But I don't think she was always unhappy. I think she had loved and relished life, and I approached her that way.

"Alex Eliot [Diane's lifelong friend and lover shortly after her marriage to Arbus] was the first person to point that out to me. He said, 'For God's sake, she wasn't negative, she was positive. She noticed everything.'

"In the end, she's an enigma. But I think most great artists -- most special artists -- are mysteries," says Bosworth. "You shouldn't explain an artist. I didn't want to explain Arbus, but I did want to tell her story."