Talk to Joan Peyser for a few minutes and you begin to believe she is Leonard Bernstein's greatest fan. Her voice becomes more lively when the conversation turns to the subject of her recent 481-page biography; the words speed up, and the vocabulary becomes more vivid.

"He is bigger than life," Peyser says. "He overwhelms.

"I think Bernstein makes things happen. He comes into a town and he changes the course of events. His sister has said that when he enters a room the heat in the room rises and that is the truth. He has this kind of electricity, magnetism, charisma -- forces that I tie in with his awesome sexuality."

And there it is -- the S-word that started all the trouble.

"Bernstein: A Biography" is Peyser's fourth book on music, her second biography of a controversial living musician (the first was "Boulez: Composer, Conductor, Enigma," dating from 1976, when Pierre Boulez was the music director of the New York Philharmonic). But the Bernstein book is different.

Bernstein has made no public comment on the book and, according to his publicists, he is not likely to. Otherwise, it has fascinated readers and enraged some commentators (a minority, Peyser insists) because it speaks frankly about the subject's sexuality -- in fact, his bisexuality. Bernstein has not made a secret of this characteristic; it is common knowledge in musical circles.

But it does raise a question when Bernstein becomes the subject of a biography. Obviously, he deserves to have his life presented in a thorough, well-researched book. In the past, it has been the custom to bypass the sex lives of most living biographees -- unless, of course, the sex life was the reason for the biography. In the case of Bernstein, Peyser would argue -- and it is hard to deny -- much of the life makes little sense unless the sexuality is taken into account.

"I think the portrayal of Bernstein in this book probably would not have done him much good if he were running for public office," Peyser says. "We wouldn't want to have such a narcissistic, self-absorbed figure as president of the United States. But knowing all this about him, I don't think it separates him from the body of artists in which he moves. He's just better at his art and better at moving in a world where there is such brutality and competition."

The book covers many subjects, some in considerably more detail, but that is the one that gets attention. If she had ignored sex, Peyser might be getting more flak about her psychological treatment of Bernstein's obsession with father figures. But when sex steps into the spotlight, people tend to focus on it.

"Some people don't understand what I mean by sexuality or why I dealt with sexuality in this book," Peyser says. "But it's a force that contributes, in combination with his remarkable musicianship, to make him the most exciting musical figure in the American 20th century. I'm not necessarily saying the greatest composer or the greatest anything -- but exciting. He is certainly the only one whose reputation has crossed the Atlantic so spectacularly; no other American composer-performer enjoys the kind of international celebrity Bernstein has."

But "Bernstein" is not a sexy book; it is a book about music by a well-known musicologist, from 1977 to 1984 the editor of The Musical Quarterly, the most respected musical periodical in America. For 20 years, Peyser wrote frequently about contemporary music for The New York Times; she is a four-time winner of the ASCAP/Deems Taylor Award for distinguished music criticism, the author of "Twentieth-Century Music: The Sense Behind the Sound" and other works. And she resents air-headed talk show hosts who treat her like a common gossip.

"If I were to string together all the sentences that deal with his sexuality, his actual sexual practices, I don't think we would have eight pages there, out of a book of almost 500 pages," Peyser says. "So I marvel at this. Some critics have said in interviews, 'Gee, I know more about Bernstein's sexuality than I ever wanted to know,' and I say, 'well, we have 470 pages left to deal with,' but the interview deals only with the sexuality. There's a tremendous amount of hypocrisy here.

"I conceived and edited a big book that came out about six months ago, called 'The Orchestra: Origins and Transformations.' It won the first prize in the humanities category from the Association of American Publishers. There was not a personal word in that book; it was a cultural history, with the orchestra as the vehicle, and it didn't get any reviews. There was no sex in that book, and therefore it wasn't reviewed. There is sex in this book, therefore it is reviewed, and the reviewers attack the sex in the book. So there's a Catch-22."

Foremost among the book's many interwoven subjects is simply the factual detail of Leonard Bernstein's life -- dates, names and places -- from his precocious Boston childhood to his latest major effort, the opera "A Quiet Place." It represents an enormous amount of disciplined labor carried out with distinction, and Peyser wonders whether critics who have never tried to write a biography understand the effort involved. "When they come in for the jugular . . . I think, 'My God, do they realize how much work it is to put all this together and make it flow?' "

Themes handled one way or another in the book include the struggle between tonality and atonality in modern music; the struggle in Bernstein's mind between European traditional forms (embodied, for Bernstein, largely in the figure of his mentor Serge Koussevitzky) and the more popular American idioms that were Bernstein's heritage; the problem of a young American conductor (particularly a young Jewish American conductor) getting a major orchestra in the 1940s; the general ferment of American musical life and how Bernstein has reflected and influenced it.

While every page is imbued with the flavor of Bernstein's personality, he is also treated frequently and not incorrectly as a representative of American music in general, as it has developed during his lifetime.

It might also be conjectured that, at some level of awareness, Peyser has chosen the subjects of her two major biographies not only because both were composers as well as performers and both were conductors of the New York Philharmonic, but even more because Boulez embodies, as much as any living musician, the spirit and ideal of atonal music, while Bernstein equally embodies those of tonality.

"You don't fully realize it when you intuitively select a subject," Peyser says, "but what has come through in this book, and what I feel is its major theme, is the intimidation by Europe of America and the fact that Americans, who had in their possession the freshest musical language of the century, the uniquely American language -- jazz -- frittered it away because of intimidation by Europeans.

"Bernstein certainly did it because of Koussevitzky's feeling about Broadway and his inability to just tell Koussevitzky to go to hell . . . he didn't say, 'Look, I believe in my own roots, in my own heritage.' I'm not saying he should have or could have done that, but it's a hell of a lot easier for Steve Sondheim to do it today because the authority figure in Sondheim's life, comparable to Koussevitzky in Bernstein's, was Oscar Hammerstein. He was his godfather, and Hammerstein did not apologize for writing for Broadway; he treated it with respect, and with technical facility.

"I think that Bernstein's major problem about his musical life may be that he wants to be 'respectable' and Broadway was not considered respectable by the people he admired."

Personally, Peyser wishes Bernstein had continued to compose for Broadway, ignoring Koussevitzky's warning that he would not get the Boston Symphony (which he didn't get, anyway) if he had Broadway associations.

"Today," Peyser says near the end of the book, " 'West Side Story' is still the single piece for which Bernstein is best known." This statement comes right after two quotes from others in which Bernstein is compared to Verdi and Mozart. Peyser would not put him on that level, but her implicit judgment is that "West Side Story" is Bernstein's best work, and many critics would agree.

In an interview, she is quite explicit on the subject: "I think his Broadway musicals will outlast many of the so-called serious classical works of his contemporaries -- American and Europeans."

As to Bernstein's opinion of the book, although he has made no public statement, Peyser insists he has talked to friends who, in turn, have talked to her. "People who have been close to Bernstein for 50 years have said to me that there isn't a nuance off base in this book," Peyser says. "So the other people can scream and shriek about the Freudianism or anything else . . .

"I have spoken to people who are close to Bernstein, so I can say I have been told indirectly that he has no problem with this book. It's been made impossible for him to begin to tell the world who he is and what he is. For him, it's not enough just to be loved; he has to be loved by people who know who and what he is; otherwise, it's a fake."

When she is asked whether she counts herself among Bernstein's admirers, the answer is immediate and emphatic:

"Yes. Absolutely. I would never spend years writing a book about someone I didn't admire. I choose someone because he's an important artist, because he's an intriguing and fascinating artist, and then I try to show why he has done what he has done, and how he has managed to do it. But I would never select as a subject of a biography someone I did not admire. Never."