ONE EVENING in the mid-1950s Mr. L., a quiet young writer, moved into his new brownstone apartment on East 94th Street, New York City. Before going to bed he opened a window and heard, floating out of the townhouse opposite, the not unpleasant sound of a nursery tune picked out on a piano with one finger, over and over again. Throughout the night in his dreams he heard the repetitions continue. He awoke to the same music, only now it was louder, the notes hammered with inexorable rhythm. He shut the window and tried to work, but finger after finger reinforced the cantus firmus across the street, adding harsh mordants, steely scales, and thunderous chords. Eventually the whole universe seemed to be vibrating with noise, whereupon Mr. L., losing control, threw up the window and screamed, "WILLYA STOP PLAYING BAA BAA #**!ING BLACK SHEEP! !"

The silence that followed was broken by the drumming of his landlady's shoes as she rushed upstairs. "How dare you! That's poor Mr. Horowitz learning to play the piano again. And it's not "Baa-Baa Black Sheep," it's Mozart's Variations on "Ah, Vous Dirai-Je, Maman!""

Unimpressed by this information, Mr. L. speedily terminated his lease; but he has ever since dined out on the story. As Horowitz apocrypha go, it's better than most, and as plausible as any, in essence if not in detail. Anybody who heard the man perform Rachmaninoff's B flat minor Sonata at the Kennedy Center last fall will remember the proliferating waves of sound, rising to such levels as to provoke disbelief, then something like fear. What tiger lurks within that sonic cage?

Glenn Plaskin, in this unauthorized biography of the world's most highly paid classical performer, gives us reason to believe that the tiger is, in part, suppressed homosexuality. This suppression, with its crippling side effects, seems to date back at least as far as 1933, when Horowitz decided to marry the formidable Wanda Toscanini, and, by extension, fall under the authority of her even more formidable father. "Toscanini is like an icon to me; and Wanda is part of the icon."

The two Toscaninis sought not only to "cure" Horowitz, Plaskin says, but also to discipline his instinctive art. He dutifully presented Wanda with a child, Sonia, in 1934, and submitted to the scourge of Toscanini's metronomic baton. Within two years a critic noticed that "at the core, his whole playing is dead." Racked with psychosomatic illnesses, Horowitz underwent "a nearly complete collapse of body and mind," and withdrew from the concert stage -- the first of several retirements, totaling some 22 years. (The pianist is now almost 80.)

One opens Plaskin's biography with misgivings, because it cannot fail to inflict pain on its subject. Since neither Mr. nor Mrs. Horowitz asked for the book to be written, and did not collaborate in the research (the jacket copy is misleading on this point), the question of privacy arises. Also of relevance. So what if Horowitz is a monster of vanity and greed? The fact remains he plays Scarlatti more beautifully than anybody has a right to expect.

It is to Plaskin's credit that, having decided (with good reason) that Horowitz's artistic and personal lives are indivisible, he proceeds to write with discretion, scholarship, and a sense of proportion. A pianist himself, he is primarily interested in Horowitz the musician. The average reader, indeed, might find the book overstuffed with the minutiae of piano technique, recording sessions, and management contracts. But this reviewer is tempted to echo Thomas Mann's "Only the exhaustive can be truly interesting."

Vladimir Samoliovich Gorovitz was born of prosperous Jewish stock in Berdichev, Russia, in 1903. He was a pampered, self-indulgent boy, musically gifted but no child prodigy. Professors at the elite Kiev Conservatory were unable to instill any reverence for Bach or Beethoven in him; anti-intellectualism proved to be a lifelong trait. Instead "Volodya" mooned over romantic opera scores, recreating them on the piano in the curious flat-fingered style he learned from his syphilitic teacher, Felix Blumenfeld. Almost without knowing how, he coaxed from the keyboard the infinite colors and textures of Wagner and Strauss, imitating brass and strings and woodwinds and drums, even the human voice. By the time he made his formal in the aftermath of the Revolution), young Gorovitz had largely perfected the most flawless technique in pianistic history.

Already, however, there was something anachronistic about his musicianship: his swooning rubatos and perfumed pedalings seemed out of place in the bleak world of Bolshevism. Yet audiences flocked to hear him, rather than the percussive Prokofiev. He even had his own fan club of girls who dressed in green.

Gorovitz was soon taken up by Alexander Merovitch, a talent-spotter and agent who also discovered the violinist Nathan Milstein and the cellist Gregor Piatigorsky. Merovitch engineered the debuts of all three in Berlin in January 1926, but Volodya's career did not catch fire until he made a sensational stand-in performance of the Tchaikovsky Concerto at the Hamburg Philharmonic. "Not since Hamburg discovered Caruso," exulted a critic, "has there been anything like it."

It was shortly after this that Rudolf Serkin first heard Horowitz (as he now spelled his name). "The young, slim, delicate-looking man sat down at the piano...I nearly fell off my chair, it was so amazing. The white heat of his playing, the fire and passion were incredible, and my hair stood on end." Serkin, a kindly soul and a superior talent, is one of the few musicians in the book who are generous to Horowitz. Rachmaninoff is another. Arthur Rubinstein is surprisingly bitchy.

On January 12, 1928 Horowitz gave an even more sensational performance of the Tchaikovsky at Carnegie Hall, literally beating Sir Thomas Beecham to the final chord in an avalanche of octaves martellato. "It was not musical and it was not necessary," growled Rachmaninoff, but the young Russian merely laughed. He had discovered how to "epaterla bourgeoisie."

For the next quarter-century Horowitz relentlessly pursued this aim, amassing huge sums of money and indulging his passion for fine cars, clothes, and paintings. Critics were divided over his interpretive gifts -- Virgil Thomson, in a famous phrase, called him "a master of musical distortion" -- but the bourgeoisie's appetite for epatement remained insatiable. He obliged with a series of coruscating showpieces, such as his arrangements of the Saint-Saens Danse Macabre and Sousa's Stars and Stripes Forever. Despite their trashy source material, these contrapuntal fantasies are the last word in pianistic ingenuity, and reveal a real gift for composition. Might the caged tiger be a creative, rather than merely physical animal?

Horowitz's mental health deteriorated in inverse ration to his frame, despite psychotherapy, medications, and a series of valets hired by Wanda. It is hard to feel sympathy for a woman so hard and cold, so relentless in her determination to make Horowitz "a king," yet Plaskin, without apologizing for her, manages to arouse it. One wonders who else could have tolerated the pianist's neurotic narcissism. Horowitz admitted that "Wanda made a man out of me."

By 1949, however, they were "fighting like Spanish fishwives," and Horowitz summoned up the courage to try life on his own. His deterioration accelerated, and in March 1953 he was brought back to East 94th Street, semicomatose. This time his retirement lasted 17 years.

In seclusion Horowitz achieved a measure of serenity, exploring deeper parts of the repertory and producing his finest recordings, for the Columbia label. These discs were supervised to the last semiquaver by Horowitz himself; with his uncanny ear and sense of the right splice he anticipated by a decade Glenn Gould's theory of editing as an art form.

He also accepted as pupils, and later discarded, seven gifted young male pianists (the relationships were purely musical). Horowitz was unable, however, to turn off his own megawattage when they played for him. Gary Graffman and Ivan Davis were the only two who escaped without radiation burn. Others, like Ronald Turini and Alexander Fiorillo, never fully realized their early potential.

The story of Horowitz's legendary return to Carnegie Hall in 1965, and the even more special, the Metropolitan Opera House recital, the Golden Jubilee concert in 1978 -- is too well known to be repeated here. Less well known are stores of further retirements, depressions, and electroshock treatments. Plaskin tells the tragedy of Sonia Horowitz, brain-damaged at 22, an apparent suicide at 40. (As Wanda struggled to recover from the death of their only child, Horowitz explained indifferently, "Her daughter died.")

One closes this book, therefore, more eagerly than one opens it. It is nevertheless a historical document, not least because it includes a discography by Robert McAlear. Plaskin's research is based on 650 interviews with an impressive variety of sources, and perhaps because of this his style can be irritatingly conversational; but he is an honest biographer. Horowitz deserves no less.