LATE IN THE YEAR 1927, as he was approaching his 40th birthday, Arthur Rubinstein finally began a long, slow, sometimes agonizing process that can best be described as his adolescent identity crisis. This would seem terribly late in some lives, but may be about right for someone who has just passed his 93rd birthday and shows no sign of being ready to slow down.

Rubinstein was a child prodigy, making his public debut at eight and his first major appearance (in Berlin, with an orchestra) a few years later. Blessed and cursed with innate musicianship, he had never been forced to exert himself unduly to win either applause or an easy living complete with wine, women, song and an occasional visit to a casino. Money came as easily as it went, and most audiences were easy to please -- particularly in Spain and Latin America, where his flair for the Hispanic flavors of Manuel de Falla and Isaac Albeniz provoked wild excitement.

If audiences, critics or concert managers were occasionally more reserved (as they were, sometimes, on his early American tours), he could easily scratch such places from his busy itinerary. Shielded from reality by uncritical admirers (including always-available women, most of whom he still recalls in loving detail) he described himself as "a confirmed bachelor" but could just as easily have been called the oldest child prodigy in the world.

Then, within a few months of one another, two encounters disrupted his happy-go-lucky life. First, he met and fell in love with beautiful, blonde, 18-year-old Nela Mlynarska, who is today Mrs. Rubinstein but did not assume that role until nearly five years later, after many reversals, including her brief marriage to another pianist.

Shortly after the first encounter with Nela, Rubinstein had another which may have shaken him even more deeply, though only a few ripples show on the multicolored surface of this second volume of his autobiography. First on a recording, then in person, he encountered young Vladimir Horowitz, who had recently escaped from Russia with violinist Hathan Milstein and cellist Gregor Piatigorsky. f

"I heard you in Kiev when I was seven years old," Horowitz told Rubinstein at their first meeting. "That was my first concert." A few days later, attending his first Horowitz performance, Rubinstein began to realize that there were depths of piano technique beyond his easy reach. They began to play duets together, privately, and he noticed that the younger pianist was condescending to him, that "he did not consider me his equal." This plunged him into "a deep artistic depression," he recalls.

"Deep within myself, I felt I was the better musician. My conception of the sense of music was more mature, but at the same time, I was conscious of my terrible defects -- of my negligence for detail, my treatment of some concerts as a pleasant pastime, all due to that devilish facility for grasping and learning all the pieces and then playing them lightheartedly in public; with all the conviction of my own musical superiority, I had to concede that Volodya was by far the better pianist."

After he had married Nela, taken her on triumphant tours of the Soviet Union and Latin America and become a father, Rubinstein began slowly to confront the problems raised by Horowitz. He rented a home in France, put a piano in the stable and began to work on his technique. It was, he recalls, "a strange musical life, a life which was completely new to me. . . . I discovered the joy of practicing." In the process, he was transformed from an overaged prodigy, getting by on uncultivated natural talent, into one of the most remarkable performing artists of the century.

This story, which is the significant part of Rubinstein's autobiography, is scattered over more than 100 pages at the center of this massive second volume. Before and after it, and frequently interrupting its course, are anecdotes and trivia, reminiscences of long-ago concerts and adventures, vivid little portraits of personalities who range from the greatest composers of the age to the women who temporarily shared his bed -- an opera singer with the temperament that folklore attributes to her kind; an Italian princess who nearly destroyed his effort to marry Nela.

Rubinstein was performing in Rio de Janeiro when the French embassy there had one of history's most remarkable diplomatic staffs: Paul Claudel and Darius Milhaud. He was present at a meeting between Stravinsky and Rachmaninov when the two composers could find nothing in common to discuss except their problems with royalties. He was the one who advised Stravinsky to become a performer if he wanted to make money with his music, and the one who secured a wealthy patron for Heitor Villa-Lobos, allowing him to live in Paris and produce his prodigious floods of music.

One aspect of Rubinstein's art that is likely to be forgotten today, when his reputation is based so solidly on recordings of the classics, is how closely he related to the composers of his own generation and how much of their music he played, sometimes in the face of serious audience resistance. Besides being a chronicle of how he was transformed from a flashy crowd-pleaser into an artist for the ages, this volume documents the creative interaction of one of music's supreme performers with some of its great producers.

But the serious values scattered through its 600-plus pages are not the book's chief selling point. It should spend a comfortably long time on the best-seller lists because it has a unique blend of charm and gusto which are deeply rooted in the personality of its author and subject. One can dip into it like a box of chocolates, nibbling here an anecdote about Ravel's self-centeredness, and there a vignette of the young, impoverished Emil Gilels seeing Rubinstein off at a railroad station on a Russian winter morning, with three roses in his hand that must have cost the price of his lunch and two of his precious fingers protruding half-frozen from holes in his glove. There are epic parties, amorous misadventures that read like a French farce, shrewd judgments of most of the notable musicians of the century, wild adventures in exotic places and discouraging encounters with dilapidated pianos all over the world.

Rubinstein avows at the outset that "this work was done out of sheer memory without the help of documentation or exterior help," but on the printed page the stories flow in a constant, colorful stream with never a moment of vagueness or faltering. Many pages have the special polish of stories that have been told over and over again, and the stories from more than half a century ago are often more vivid and detailed than the recent ones -- perhaps because he has told them so often.

It may be that some of the anecdotes have gained through retelling, or that Rubinstein's imagination has come to the rescue occasionally when memory has lapsed. But such lapses are hard to find and largely insignificant in the Rabelaisin bulk of the work. Whatever may be the verdict of scholars on this primary source for their studies, the general reader will find Rubinstein's memoirs one of the most enjoyable autobiographies of our time. CAPTION: Picture 1, Arthur Rubinstein; Copyright (c) by Avraham Toren; Picture 2, Arthur Rubinstein