WE DEAL with a phenomenon--a composer never far from a keyboard who is also a writer seldom out of range of a mirror. The keyboard has been the source of symphonies, operas, and scores of the most literate songs of the century. From the mirror have come reflections of the writer's passage from the days in Paris when he was Narcisse incarn,e in an illustrious era of la jeunesse dor,ee to nowadays Nantucket where, ,eminence grise malgr,e lui, he casts a sage cool eye on the musical scene and endures the Daumier-like pleasures of domesticity.

To chart Ned Rorem's passage in Gallicisms is merely appropriate. Among the many American creative talents who have embraced the shibboleths and aesthetic modes of French culture, he seems less to have adopted these than to have magically inherited them. Born in Indiana of Quaker parents, he grew up in Chicago where, one day, the music of Debussy "opened the gates of heaven"--an experience which meant membership, however remote, in the garde which, still avant in the heyday of Les Six and the breakthrough of Schoenberg's atonal Serenade, enlisted his adolescent sensibilities and set the direction his career would take. Instant identification with a theretofore hidden world made him a kind of avatar; yet Rorem's conversion did not obscure a back-home- in-Indiana touch of the yokel in him which gives his diaries their gaga candor and provides their invitations to voyeurism.

Begun more than three decades ago, published in the mid-'60s and now reissued in one volume, they have outlived the thousand other more sensational and more clinically detailed confessional volumes which have meanwhile appeared. The difference is that Rorem's diaries are more concerned with reaction than with action, more with the consequences of egocentrism and satyriasis than with their pathologies, and simply because they are better written. Yet, praise be, they are not "well-written." The French knack for verbal economy and razory discrimination is not one of this diarist's inheritances. Extravagant and repetitious, mixing apercus with windy self-analyses and gropings toward epigram, Rorem has the luck of the inspired amateur who hits all but the targets at which he is aiming. These accounts of his salad days offer the raw stuff of feeling, the tantrum no less careless of discretion than the trauma. Like one's response to the bad child's blurtings of unguarded truth, part of a reader's fascination with this chronicle of homosexual cruising, alcoholism, and elbow rubbing with the rich and titled is the exasperation it evokes, the impulse to tell the kid both to shut up and go on. What is he going to say next?

The perhaps unexpected element in Rorem's memoirs of those years in Montmartre and Manhattan where he seemed to know everyone but himself is generosity. When there are pretensions to be exposed and scores to be settled, he begins with his own. When his opinions are strong--like those about his bete noire Boulez ("If Russia had Stalin and Germany had Hitler, France still has Pierre Boulez")--we learn why he holds them. And just when his devouring impatience with the pieties and institutionalized cliches of musical history reaches a point where we suspect he is giving us sour grapes instead of judgments, he puts doubt at rest with observations informative and compelling.

Yet his innate charity is not without limit. The scorn he heaps upon those agencies and academies which have made art a commodity, or upon those theories and methods by which art is supposed to be taught on premises barren of mystery, is relentless. Adroit and recondite, his running attack is powered by something never mentioned, even by an author for whom self-conciousness is a primary theme. The truth is that Rorem is a natural, one of those chosen few--among artists as often as among their explicators--whose early experience of art was absolute and ineffable, a conversion the reality of which would ever after be lost for words and yet dependent upon them. Compared with those more orderly critics who often sound as though they were preparing briefs for what they are by temperament permanently denied, Rorem speaks from a still center of knowing, undisturbed by exigency.

Essays and a Diary as subtitle for the new book is only nominally accurate and may mislead readers expecting a continuation of the extraordinary life to which the earlier diaries introduced them. Some 90 percent of the reviews and essays in Setting the Tone have to do with music and musicians, teachers and performers. In these thoroughly professional discourses, the self-indulgence and logorrheic flow of the diaries is curbed. Perceptions that evade the author when he talks about himself are at his fingertips when he discusses his forebears and contemporaries.

On Ravel: "In 1875 he was born (on Jane Austen's hundredth birthday) of well-off and understanding parents in the village of Ciboure near the Spanish frontier. These few facts illuminate all that he became. His art straddled the border as it straddled centuries, being in texture as opulent as a tourist's notion of Iberia, in shape as pristine as Rameau, in intent no less modern than ragas or group therapy and in subject matter mostly antiromantic. Listen again to Bol,ero. ('It's my masterpiece,' said the composer. 'Unfortunately it contains no music.') French logic drenched in Basque mystery."

On The Beatles: "The Beatles have, so to speak, brought fiction back to music, supplanting criticism. No, they aren't new, but as tuneful as the thirties with the same exuberance of futility that Bessie Smith employed. They have removed sterile martyrdom from art, revived the sensual. . . If (and here's a big If) music at its most healthy is the creative reaction of, and stimulation for, the body, and at its most decadent in the creative reaction of and stimulation for the intellect--if, indeed, health is a desirable feature of art, and if, as I believe, the Beatles exemplify this feature, then we have reached (strange thought it may seem as coincident with our planet's final years) a new and golden renaissance of song."

Of interest mainly to the initiated, these pieces nevertheless contain enough general information to entice the lay reader. For readers to whom serialism, atonality, and tonic-dominant harmonies are mysteries approaching the Eleusinian, Rorem's easy and irreverent approach to the polemics of 20th-century musicology may be a comfort. In fact, Rorem as critic is so much in control of himself and his materials as to suggest that the roaring boy of the diaries may not, after all, be a case history but a creation as artfully limned as Dorian Gray.