STEVE CAUTHEN, ONLY 18 AND RIDING HIGH, ALREADY HAS HIS OWN BOSWELL

So much has been written about Steve Cauthen that nothing of moment can be added here. But it ought to be noted that Cauthen, at the ripe old age of 18, has already found his Boswell.

He is Pete Axthelm, the author of The Kid (Bantam, $2.50), a paperback biography rushed into print ahead of schedule on the heels of Cauthen's splendid spring victories at the reins of Affirmed. One might assume that the book is a quickie, but it isn't; The Kid is a solid, sensitive piece of work that does credit both to its subject and to its author.

Though Axthelm is perhaps best known for his pioneering book about urban basketball. The City Game, his journalistic roots are planted in the racetrack. He has been a railbird for years, and he brings all his knowledge of the racing world to bear in his account of Cauthen's brief, astonishing career. The Kid is not just another breathless paean to a sports celebrity; it is also an intimate, humorous, anecdotal exploration of the little universe in which the horse is king.

Axthelm spent a lot of time with the Cauthen family in the course of preparing the book (it is an "official" biography, and he and Cauthen share the royalties), and he has gotten fascinating, often touching material about these honorable, unpretentious, hardworking people. As Tex Cauthen, Steve's father, told Axthelm, "I guess the corny way to put it is that horses are in our blood." Tex and his wife Myra are horse people who have made respected careers out of the work they love most; Steve is the most sensational result of their labors.

Readers and TV watchers know that Cauthen is not exactly a great interview, but Axthelm persuasively portrays him as a bright, level-headed kid with a firm sense of where he came from and where he fits in. Axthelm devotes much space to Cauthen's uncanny feeling for horses, his diligence and self-discipline, and his sense of racing tactics. The Kid, in Axthelm's word, is a natural.

So is the book. Axthelm tells page after page of funny, revealing stories, and he writes with affection and perception. "The beauty of racing," he says, "lies largely in the fact that almost any race can be described with equal accuracy and excitement by saying 'He paid eight dollars' or 'He fulfilled somebody's dream of a lifetime.' " On both counts, that is the story of Steve Cauthen.