HGifted sportswriter Hillenbrand unearths the rarefied world of thoroughbred horse racing in this captivating account of one of the sport's legends. Though no longer a household name, Seabiscuit enjoyed great celebrity during the 1930s and 1940s, drawing record crowds to his races around the country. Not an overtly impressive physical specimenD""His stubby legs were a study in unsound construction, with huge, squarish, asymmetrical `baseball glove' knees that didn't quite straighten all the way""Dthe horse seemed to transcend his physicality as he won race after race. Hillenbrand, a contributor to Equus magazine, profiles the major players in Seabiscuit's fantastic and improbable career. In simple, elegant prose, she recounts how Charles Howard, a pioneer in automobile sales and Seabiscuit's eventual owner, became involved with horse racing, starting as a hobbyist and growing into a fanatic. She introduces esoteric recluse Tom Smith (Seabiscuit's trainer) and jockey Red Pollard, a down-on-his-luck rider whose specialty was taming unruly horses. In 1936, Howard united Smith, Pollard and ""The Biscuit,"" whose performance had been spottyDand the horse's star career began. Smith, who recognized Seabiscuit's potential, felt an immediate rapport with him and eased him into shape. Once Seabiscuit started breaking records and outrunning lead horses, reporters thronged the Howard barn day and night. Smith's secret workouts became legendary and only heightened Seabiscuit's mystique. Hillenbrand deftly blends the story with explanations of the sport and its culture, including vivid descriptions of the Tijuana horse-racing scene in all its debauchery. She roots her narrative of the horse's breathtaking career and the wild devotion of his fans in its socioeconomic context: Seabiscuit embodied the underdog myth for a nation recovering from dire economic straits. (Mar.) Forecast: Despite the shrinking horse racing audienceDand the publishing adage that books on horse racing don't sellDthis book has the potential to do well, even outside the realm of the racing community, due to a large first printing and forthcoming Universal Studios movie. A stylish cover will attract both baby boomers and young readers, tapping into the sexiness and allure of the ""Sport of Kings."" Hillenbrand's glamorous photo on the book jacket won't hurt her chances, and Seabiscuit should sell at a galloping pace.