Bernstein -- readable and candid; Findings, Leonard Bernstein. New York: Simon & Schuster. 375 pp. $17.95.

I picked up this book with only slight interest in Leonard Bernstein, a great interest in music, and a profound cynicism of ''celebrity memoirs.'' And I was pleasantly surprised.

As a composer, conductor, and pianist, Bernstein is considered one of the most versatile, eclectic, and truly American among contemporary musicians. This pastiche of poetry, essays, speeches, musings, and critical studies is more than a self-serving, print-version mirror of that versatility. Bernstein has focused, for the most part, on material that reaches broadly beyond personal realms to embrace the serious concerns of music itself: the future of the symphony, conducting vs.composing, programmaking, and critics.

Even in the strictly personal anecdotes - the overriding concern seems to be with reinforcing people and qualities that bolster the tone of art in all its forms. Bernstein has led an exciting life of discovery in music. His many speeches, his young people's concerts, his flamboyant conducting style, and this book all reflect his zeal to share that discovery. If slow to accept his invitation at first, you gradually become willing to go along.

Excerpts from a ''West Side Story Log,'' which Bernstein began eight years before opening night, note that the choreographer, Jerome Robbins, suggested a modern version of ''Romeo and Juliet'' set in slums at the coincidence of Easter-Passover celebrations. Six years later they abandoned the Jewish-Catholic premise and replaced it with two street gangs: ''one the warring Puerto Ricans, the other self-styled ''Americans.'' The log culminates the day after the opening: ''Washington, D.C. Aug. 20, 1957. The opening last night was just as we dreamed it. All the agony and postponement and re-rewriting turn out to have been worth it. There's a work there; and whether it finally succeeds in Broadway terms, I am now convinced that what we dreamed all these years is possible. . . .''

Here also is Bernstein's testimony before a House subcommittee regarding a White House Conference on the Arts. Bernstein makes clear he feels that no amount of government funding for the arts will alter what he feels is a fundamentally uncultured nation. He appeals for the musical instruction of every citizen from early school on. This is how, says Bernstein, a truly alive, receptive, and critical national audience could be fostered, which would demand and support the best that artists can supply.

Part of Bernstein's success is that he knows when to not take the world, the musical establishment, or himself too seriously. He is often self-mocking, self-effacing, constantly apologizing for talking on too long in his speeches or not getting to the point. His self-effacement tends to explode that hyped image of the snobbish musical supersophisticate.

As one reads his Harvard bachelor's thesis, ''The Problem of Nationalism in American Music,'' one finds he is surprisingly erudite for a 21-year-old undergraduate. He elaborates on the nature of nationalism in music itself and traces the outgrowth of American nationalism in Puritanism, Negro, and American Indian music as well as from German, English, and French traditions.

Bernstein's writing is readable, always candid, and often funny. His account of his last visit with Nadia Boulanger, essays on ''Fun in Art,'' ''Mahler: His Time Has Come,'' and a poem entitled ''The Israel Philharmonic Blues,'' are three more standout examples of an amazingly perceptive, talented man imparting an ardor for life in music. Few else have succeeded as well.