Justin Kaplan, who's just published a critically acclaimed biography of Walt Whitman, smiles faintly -- and a little mockingly -- when recalling the intimidating stares of several scholars while working on his first book, the Pulitzer-prize-winning biography of Mark Twain.

He remembers it as a risky venture, stepping into the Mark Twain Collection at the University of California at Berkeley.

"There was a room full of scholars." he says in an evenly-paced nasal tone."They looked at me as if I were some sort of misplaced apparition."

Switching to an arch vocal inflection, he continues: "'Imagine the nerve of someone coming here and spending only two months going through the papers and the next thing he's going to go back home and write a biography. Whereas, 've been working on this for 20 or 25 years or so on,'

"Some of the people made me feel uncomfortable, but really the good people in the field were wonderfully hospitable."

Still, what he describes is a growing phenomenon in American letters. Like knights locked in mortal battle, biographers and scholars sometimes joust over the literary remains of an historical figure -- for rights to who's going to write the life story.

After the death of a famous novelist, composer, scientist or politician, the scholars habitually divide up the person's life and become expert at specific, sometimes esoteric, aspects -- The Significance of 19th-Century Swiss Educational Reform on Albert Einstein's Student Ears, for instance.

"Look, I'm a writer-biographer," protests Kaplan, "and I happen to be very deeply interested in Whitman or Twain, and I'm going to spend the next few years doing a book, which means I'm mastering most of the scholarship.

"And then after I've done that I'm going to move on to somebody else. It doesn't mean that writer-biographers are hit-and-run operators, but we're writers. And the academics tend to resent that. We've muscled in on their areas. We've creamed their subject. We've destroyed a lot of the arrangements they've made over the years."

Kaplan, 55, a compact man of small build, was wearing a splash of colors that came together ingeniously -- a rust sport coat, an orange tie backgrounded by a navy cotton flannel shirt and gray slacks. He had just returned to his hotel from watching President-elect Reagan at 1715 Kackson Pl. NW.

"I had read in the paper where he was going to be," Kaplan says, "and I thought I ought to have a look at him. I didn't vote for Reagan. I had never seen a president in a brown suit before. Not a tan suit, but a really brown suit."

Kaplan has received almost unanimous praise for his biography of Whitman, frequently called the greatest American poet, the writer whose very life was considered a gradiose poem.

Saturday Review called the book a "delicate, calm and painstaking biography." The Washington Post deemed it "a handsome narrative, invariably fluent and intelligent." And from The New York Times: "Justin Kaplan's taste is very nearly impeccable, and by his emphases and omissions a critical view of Whitman's best work is silently converyed; and Mr. Kaplan's human insight makes psychological connections that illuminate, often by delicate juxtapositions of texts, a whole tract of poetry."

On the strength of his previous two biographies -- of Twain and journalist Lincoln Steffens -- Kaplan was already considered a major American biographer, master of a form which is gaining wider literary recognition.

"The point I've been making year after year is that biography is not just some guy sticking together a lot of research and facts and scholarship and finding a box or package for it," Kaplan says. "It's as literary a form as the novel or a play or a poem. It's an act of writing, not of library work -- although you have to do your homework.

"Ten or 15 years ago the main emphasis in a review of a biography would have been purely on the substantive content. Now you have critics reveiwing biography who are very respectful about the book, and they pay a great deal of attention to how the book is written."

Kaplan's decision to write biography was almost spur-of-the-moment. After undergraduate and graduate work at Harvard, he worked as a freelance editor in New York and at one time assisted anthologist Louis Untermeyer. He thought he'd like to write books, maybe history. But he wasn't sure.

"One day I was having lunch with a close friend and a mentor," he recalls. "We had this publishing lunch, which means a long boozy lunch. He said, 'Why don't you write a book about Mark Twain?' It never would've occurred to me in a million years to write a book about Mark Twain. But somehow there was some electric reponse.

"I'm not the kind of person who makes impulsive decisions. I thought about it for 15 minutes and went across the street to Doubleday Book Shop and bought whatever they had of Mark Twain. I spent the next two weeks of Simon and Schuster's time compiling a bibliography, a work plan and outline. I really decided that, yes, I was going to leave publishing and write this biography. And it was not just the subject of Mark Twain but the idea of writing biography.

"I find biography a congenial form. It's ideal. I really want to stick with it. I've been asked to do various kinds of histories. I really don't want to do that. I like writing about people."

Kaplan grew up reading about people. He remembers his father, a shirt manufacturer who really wanted to be a scholar, doing a lot of reading. "I was brought up on Tolstoy and Boswell's 'Life of Johnson' and Pepys' diary," he says, smiling contentedly. "And for a while I hated both Johnson and Pepys because they had been hurled at me throughout my childhood."

His mother died when he was eight and his father died six years later.

"It's been so long since I had parents that I tend to think I never had any parents," he says. "I was lucky because I was brought up by two people: my brother, who was nine years older than me; he died last year and he's the one to whom the Whitman book is dedicated, and then a West Indian woman who had come to work for my mother when I was about a month old and who has really brought me up. She lives in New York now. I see her often.

"It was a very loving and reinforcing kind of household. My brother very wisely thought I ought to go away to college. He had osteomyelitis and spent two or three months out of every year being operated on at Mt. Sinai Hospital. But he really wanted me to have everything. He never put a leash on me.He was very patient, even when I was horsing around.

"I think of myself as a professional orphan. My father left me with enough money so that I could go to college and do a certain amount of traveling and not have to worry right away about a paying career."

So after college and some travel in Europe, he tried his luck in the New York publishing world, going from publishing house to publishing house on a freelance basis. After settling in at Simon and Schuster, he became editor for Bertrand Russell, Will Durant and Nikos Kazantzakis. But the feat he was most proud of was bringing sociologist C. Wright Mills to the company.

For the last 21 years, Kaplan has lived in Cambridge, Mass., with his wife of 26 years, Anne Bernays, also an author who's written six novels. They have three daughters, ages 23, 21 and 18.

Kaplan has just started researching a biography of Ulysses S. Grant, and he notes his fascination with Grant's era: "I suppose I'm not only in love with biography," he says, "I'm in love with the 19th century. For biography it's a terrific time because it's just before the telephone comes in. And it means that the talking and ideas and the relationships that now disappear into nothingness over the telephone were then put into letters or diaries or recorded conversations.

"You get much more intimate documentation of the inner lives of people than you do in the 20th century, even though we now have an emphasis on total candor and let it all hang out. Nineteenth-century people, without knowing it, tended to let it all hang out because sometimes they were not aware of what they were saying. . . Freud has not just made us more open about things; he's also made us more covert.

"A 19th-century person would have very little hesitation in telling his or her dreams they'd had the night before. Well, before any of us goes around talking about the dream we had the night before we do a little bit of checking to find out how really intimate that dream was. What's it going to tell other people?"

Kaplan already emits a glow while talking about his Grant project.

"He's very much like Whitman and Mark Twain," he says. "There's something in the lives of these three people that you could only describe as the quality of theological grace, that is, something very big is happening to them. Their successes are larger than the successes of normal people. Their failures are horrible failures. Mark Twain goes bankrupt. Grant goes bankrupt. He's a drunk. He's a complete- - - -up as a president."

But there was something redemptive in Grant's life, Kaplan says. Near the end of his life he quickly wrote his memoirs in what Kaplan described as a "marvelous prose style." After Grant's death, Twain arranged to have them published and the big contract for the book lifted the Grant family out of bankruptcy and assured them of financial security.

The style and pace of 19th-century life, or many other parts of the past, for that matter, are hard to convey to today's reader, according to Kaplan. "The standard language of allusion, biblical language, the language of Shakespeare," he moans, "it's like talking Chinese to people now. They will not understand even the simplest biblical reference. I mean readers in their 40s and 50s.

"For example, the other day my 23-year-old daughter asked me whether Cain killed Abel or if it was the other way around. Good God!"

Would he consider writing a biography of a living person? Kaplan thinks not. It's too hard to keep emotional distance, he says. "Also, what you can't do is write a bang-up death scene," he laughed.

Still, he says he might be interested in writing Frank Sinatra's biography.I'd be fascinated by him," he explained, chuckling all the while. "He's a very public figure of great accomplishment, and he's also very remote, not to say inaccessible -- and even a little threatening. I don't want to get my legs broken up."

Though he's working on Grant now, Kaplan hasn't put Whitman in the background yet. He likes to think of how the poet still speaks to contemporary Americans -- through his own poetry and that of poets like Allen Ginsberg and the late Frank O'Hara.

"I think Whitman, the subject, is somehow very timely and very immediate," says Kaplan, "and people are just beginning to realize that he is really a major American figure. There're certain moving things about his life, which could move a lot of other people -- a sense of potential and possibility in the most ordinary or mediocre lives.

"The whole Whitman background -- or foreground -- appears to be unpromising. He's a carpenter,j a school teacher, an okay journalist, but he really doesn't have the persistence. He runs a stationery store for a while. He moves from job to job. And then suddenly at the age of 34, something remarkable happens to him.

"Before then he'd written very conventional poetry. Suddenly he knows what he was born for and begins to write truly remarkable and original poetry. You can talk about this change in terms of ecstatic experience or some kind of illumination. But I think it's still relevant to anyone.

"What it suggests is that there're whole uncapped reservoirs of creativity in the most unlikely people. He respects what happened to him as a kind of sacred mystery. And I do, too, because you can't find it. Finally, all you can say about it is that it happened."