LEONARD MOSLEY'S Disney's World tells of a man who never put away childish things. Model railroad trains ran through Walt Disney's garden, and he had a soda fountain in his screening room. All his life, he played and dreamed. But with a difference. What he dreamed came to be.

The story has elements of Horatio Alger. The winter he was 10, Walt Disney rose at 3 a.m. every morning to deliver newspapers in the cold suburbs of Kansas City, a job that left him so tired he then slept through school. At home, he drew pictures of animals, only to have his judgmental father tear them up.

The year he was 17 (he was overseas with the Red Cross in World War I), his girl married somebody else; and his dog died. He took the dog's death harder.

A visionary, Disney's greatest struggle was to find people who shared his visions. (Recently a 1935 cartoon on a celluloid strip -- it showed Mickey Mouse leading a band -- sold at Christie's auction house for $24,200, but when Walt made Steamboat Willie in 1928, he had to sell his car in order to get the money to have the picture scored.)

Disney went to sound when nobody thought cartoons should have sound, he went to color, with his brother (and business partner) Roy fighting him every step of the way. And all the time he was working on his first full-length animated feature, Snow White and the Seven Dwarfs, he had to endure not only Roy's pleas to stick with Mickey Mouse, who was their bread and butter, but the gibes of other movie makers. Louis B. Mayer (familiarly referred to as Louie by Mosley) is said to have sneered, "Who'd pay to see a drawing of a fairy princess when they can watch Joan Crawford . . . for the same price at the box office?"

None of this affected Walt Disney. He stayed at his drawing board, inventing characters, putting them in scenes; then acting out those scenes for the artists who would animate his fantasies.

Snow White made millions, but Disney had failures too. Their first times around, Bambi, Pinocchio and Fantasia all died at the box office. Walt Disney went on working. When he turned to movies with live actors, he made stories like The Swiss Family Robinson and Mary Poppins. They were stories he had loved, and other people loved them too. When he tried television, Davey Crockett, King of the Wild Frontier and The Mickey Mouse Club were equally popular. So was Disneyland, though his own studio had so little faith that the board refused to back the amusement park, and Disney had to go out and raise the funds himself.

He was anti-Semitic, anti-black, and he hogged screen credit. And when his employes struck for a union (he had kept 2,000 of them working through the Depression), he felt so betrayed he took away their morning cookies and their volley ball nets; and he put in a time clock.

In the early years he was often broke, but he was never broken. And at the end, dying of cancer, he was still furiously planning a utopian city of the future. (Nobody is sure he won't yet come back to oversee it. There are rumors that Disney had himself frozen by cryogenics, against the time when medical science would be able to fix what ailed him.)

THERE HAVE BEEN many books about Disney's life and/or work, but until this one, I had read none of them, so I can't tell you how much of Disney's World is new material, and how much Leonard Mosley owes to previously published volumes.

In fact, this book is oddly footnoted. Once in a while at the bottom of a page, we find "Disney Archives, Burbank," or "In conversations with the author"; at other times, there is no attribution whatever.

For instance, after Walt Disney made Alice in Wonderland, Mosley writes that Roy Disney called it "intellectual crap," and yelled, "You only made it to puff up your . . . ego." Enthralling, but undocumented.

I have other quibbles. Sloppy writing, for one. Mosley says Walt's press agents painted Disney as perfect, but Walt "was not, after all, the whiter-than-white sepulchre the studio propagandists made him out to be." The truth, says Mosley, is that Disney was "a far from perfect and therefore much more fascinating man."

Mr. Mosley, dear, a whited sepulchre isn't perfect, it's nasty. ("A hypocrite who conceals wickedness under a cloak of virtue," according to Brewer's Dictionary of Phrase and Fable.)

Also, we get plenty of amateur psychoanalysis; along the lines of, "He saw himself .. as a weakling who couldn't even match up to the ordinary American male and produce a child of his own."

Well, Walt Disney was not the ordinary American male, he was sui generis, and it's probably dangerous for a run-of-the- mill journalist to attempt to psychoanalyze a genius. But even an imperfect book about such a rare bird can be interesting, and this one is.