Why do preppies transfix Americans? We grab for details about their lives as if they were Triscuits at a tailgate picnic. Books about prep schools, from John Knowles' "A Separate Peace" to J.D. Salinger's "The Catcher in the Rye" to Louis Auchincloss' "Rector of Justin," enthrall us. Lisa Birnbach's "The Official Preppy Handbook," which has been on top of the Washington best-seller list for four weeks, has no pretentions to literary merit. Unabashedly, Birnbach reveals how to be prep in mind and body.

She asserts that "in a true democracy everyone can be upper class and live in Connecticut." Even if you are not to the prep tradition born, she says, armed with the correct jargon and clothed in pastels, you can "pass" for a Cabot or a Lodge without shame or apology.

And it seems that every public school kid with a hyphen between his heritage believes her. Nationally, the book has sold 550,000 in less than six weeks. But I wonder whether, after reading Birnbach's book, they still want to be prepsters. Sure, members of America's ruling class are goodlooking, athletic and rich, but they are also uproariously silly.

With perception and wit, Birnbach traces the personas of prep, beginning with the baby who is swaddled only in 100 percent cotton diapers fastened with a gold kilt pin, and ending with the alumni fund-raiser, dressed in "green poplin pants with cute yellow tennis rackets" on them. She covers such vital details as nicknames -- Muffy and Skip are the most favored -- and reveals how the ubiquitous "Mummy" decorates the prep abode, giving particular attention to the duck motif ever present where preppies gather, because mallards suggest "hunting, water, Maine -- all the things worth thinking about."

Boarding school and college represent the zenith of the prep existence, and the chapters on these subjects are the most entertaining. Better than any Junior League seminar, the book describes the complete cycle of prep school, from interview to curriculum to food all the way to getting kicked out. For the last item, she provides a compendium of "booted" prepsters -- "real-life Holden Caulfields." For instance, the adolescent Bette Davis left Northfield in 1924 apparently when she was spotted waltzing over to a nearby boys' school after curfew. No surprise then that she later won an Oscar for portraying a tempestuous deb who wouldn't wear white to the cotillion in "Jezebel."

Birnbach is not content with depiciting just the obvious bastions of preppiness like Andover, Groton and Miss Porter's; she also describes such unique hybrids as the "bohemian prep school" where "students have long fuzzy hair and long fuzzy sweaters. Both have leaves in them."

For the collegiate preppie, the handbook proffers advice on selecting a major. English and history are prep, but linguistics and comparative literature "imply all sorts of unattractive qualities like intellectualism and curiosity about the universe that a self-respecting Prep wouldn't touch with a ten-foot length of ticker tape." Sports are accorded the respect due an activity which comprises 90 percent of the prep's waking moments. Crucial distinctions are drawn, however. Preppies are deeply about crew, squash and lacrosse and not at all for wrestling, track and real college football. For the ersatz kind of football -- Harvard versus Yale, or Williams versus Amherst -- they care more than about life itself.

Sex, the other undergraduate pastime, is a "contradiction in terms" when performed by preppies. According to the handbook, the prep woman craves "an escort, a Ken doll, a trophy . . . who will never lay a hand on her except on the dance floor." Meanwhile, the prep man desires "nothing less than perfection -- as embodied in his mother." Since Ken dolls and underaged mummies do not exactly suggest "the Harrad Experiment," the preppie shunts aside libido in favor of such restful activities as the layering on of Lacoste shirts.

Mail ordering L.L. Bean hunting regalia and Talbot golf skirts plays a far more significant role in prep life than sex, so Birnbach devotes many pages to the intricacies of hot pink cable-knit cardigans and needlepoint belts sporting spouting whales. Prep dressing offers a unique sinse of reasurance because there are no exceptions to any of the rules. Velour is never prep; men wear only boxer shorts; women wear what mummy wore.

No matter how you feel about preppies, you can't help liking Lisa Birnbach's book. While precisely chronicling what preppies wear, think and do, she deftly mocks the entire mystique. "The Official Preppy Handbook" is the type of paperback that everyone picks up, flips through, exclaims "can you believe people buy this," and promptly buys "for a friend." The gift is eventually delivered, but only after it has been well-thumbed.

Perhaps Erich Segal, author of "Love Story," summed it up best when he claimed the term preppie comes from the word preposterous.