A nearly 400-page biography of a horse? Well, Seabiscuit was no ordinary horse: in 1938 he reportedly received more newspaper coverage than Roosevelt, Hitler or Mussolini. His match that year with his arch rival, War Admiral -- which pitted racing fans from the West Coast against racing fans from the East -- was one of the most widely followed stories of the year and one of the decade's biggest sports events.

''The whole country is divided into two camps,'' a 1938 San Francisco Chronicle article read. ''People who never saw a horse race in their lives are taking sides. If the issue were deferred another week, there would be a civil war between the War Admiral Americans and the Seabiscuit Americans.''

In telling the Cinderella story of Seabiscuit and his devoted trainer, owner and jockey, the author, Laura Hillenbrand, has written an absorbing book that stands as the model of sportswriting at its best. A contributing editor at Equus magazine and a longtime writer on thoroughbred racing, Ms. Hillenbrand gives us a visceral appreciation of that sport as refracted through the tumultuous lives of Seabiscuit and his human companions, while at the same time creating a keenly observed portrait of a Depression-era America bent on escapism and the burgeoning phenomenon of mass-media-marketed celebrity.

Although Seabiscuit was descended from the great Man o' War, he was a supremely undistinguished looking racehorse. His boxy, diminutive body, Ms. Hillenbrand writes, ''had all the properties of a cinder block'' and his ''sad little tail'' was barely long enough to brush his hocks. His legs were stubby with ''baseball glove'' knees that didn't quite straighten all the way, giving him an odd straddle-legged walk and a spastic sort of gallop that resembled the speeded-up waddle of a duck. While most horses sleep standing up, Biscuit was given to conking on out on the floor of his stall and snoozing for hours at a time; the only thing he liked to do more was eat, a propensity that gave him an ongoing weight problem.

As for Biscuit's personality -- and Ms. Hillenbrand proves herself as adept as the best children's book writers at delineating the temperaments of the animals in her story -- it was extraordinarily easygoing, at least off the track. His first trainer would remember him acting like an amiable ''big dog''; as a colt he often seemed lazy and slow, taunting his riders, it seemed, to give him an incentive to run.

The trainer Tom Smith, however, immediately glimpsed something special about Seabiscuit, and he resolved to get his employer, Charles Howard, to buy the ungainly horse. The two men emerge as larger than life personalities: Howard, a self-made magnate who had journeyed west to make his fortune and found his calling selling Buicks to a turn-of-the-century California still skeptical of automobiles; and the taciturn Smith, a frontiersman of a very different sort: a jack of all trades and former mustang wrangler whose horsemanship was honed at rodeos, ranches and small-time race tracks.

Forming the third leg of the Seabiscuit triangle was the jockey Johnny (Red) Pollard, a scrappy veteran of the lawless bush tracks, where other riders, Ms. Hillenbrand writes, ''could -- and would -- do anything to win.'' With his offbeat sense of humor and his passion for literary aphorisms, Pollard earned the bemused affection of other jockeys, but spent years struggling to make ends meet. ''He had no money and no home,'' Ms. Hillenbrand writes. ''He lived entirely on the road of the racing circuit, sleeping in empty stalls, carrying with him only a saddle, his rosary, and his books: pocket volumes of Shakespeare, Omar Khayyam's 'Rubaiyat,' a little copy of Robert Service's 'Songs of the Sourdough,' maybe some Emerson, whom he called 'Old Waldo.' '' He would keep the fact that he was blind in one eye a secret throughout much of his career.

Under Smith's strict but sensitive training regimen and Pollard's intuitive hand, Biscuit overcame an initial spate of bad luck to become a ''fanatically competitive'' winner, picking off rivals as he set new records for speed. But if the horse had become ''a public and media darling in the West,'' Eastern racing circles were still reluctant to take him seriously. To win the respect of the racing establishment, he would have to beat the 1937 Horse of the Year -- War Admiral, a high-strung, temperamental beauty who was ''the picture of exquisite, streamlined elegance, light and fine and quick.''

Ms. Hillenbrand narrates the Seabiscuit-War Admiral showdown with the same panache she brings to the accounts of Biscuit's remarkable comeback from a seemingly career-ending injury (paralleled, uncannily, by Pollard's efforts to recover from a series of debilitating injuries) and his repeated attempts to win the coveted Santa Anita Handicap.

Although this book is marred by occasional patches of purple prose -- ''Red Pollard was sinking downward through his life with the pendulous motion of a leaf falling through still air,'' Ms. Hillenbrand writes of the jockey's travails -- it remains a first-rate piece of storytelling, leaving us not only with a vivid portrait of a horse but with a fascinating slice of American history as well.