Recently American readers have been partial to ''how to'' books such as those explaining how to achieve thin thighs quickly or sexual ecstasy slowly. But suddenly this summer -- summer: the season for spilling Coppertone on Danielle Steel novels -- there is an astonishingly different best seller. It is Allan Bloom's ''The Closing of the American Mind.'' Readers taking this book to the beach are going swimming with Nietzsche, Heidegger and others.

Bloom's subtitle is ''How Higher Education Has Failed Democracy and Impoverished the Souls of Today's Students.'' Revenge is, indeed, a dish best eaten cold, and this book is, in part, Bloom's delayed revenge against academics who found no moral resources for resisting the 1960s' mobs that broke universities to the saddle of ''relevance,'' meaning the political passion of the hour. But Bloom, a political philosopher at the University of Chicago, is really refuting the entire intellectual tradition that brought on the 1960s. This tradition is, he says, responsible for mankind's ''300-year-old identity crisis.''

Bloom and a few kindred spirits are resisting the triumph of relativism and intellectual egalitarianism. To the modern mind, those are related moral imperatives. Relativism is considered a requirement for a free society because the only modern sin is intolerance, and intolerance results from denying that all ''values'' are of equal dignity.

Relativism, says Bloom, extinguishes the purpose of education, which is the search for the good life. Democracy needs education that produces people with the knowledge, habits and character necessary for democracy. But when tolerance replaces natural rights as the basis of democracy, then ''going with the flow'' replaces rules developed by reason. These are rules for living in accordance with natural rights -- that is, in ways that are right for creatures of our nature.

''Commitment,'' says Bloom, is a word invented to serve modernity, which assertsthe absence of any natural motives in thesoul for moral dedication. What modernity values is ''authenticity,'' meaning intensityof commitment to whatever ''values'' onehas picked from the unlimited cafeteria of choices.

Today, students are taught that there is no hierarchy of choices establishable by reason. The social sciences teach this leveling lesson: the world is a bazaar of cultures, no one of which can be demonstrated to be superior to another. True, some cultures place high value on tolerance, but relativism teaches that a preference for tolerance is as arbitrary as any other preference.

Openness -- to experience, to arguments -- used to be an instrumental virtue valued because it made possible the quest, through reason, for knowledge of the objectively good. Now openness is not an instrument; it is an end. Indeed, it is the only universal value, reason having been declared powerless to discern the good. But there is vanity beneath the intellectual humility: openness makes the absence of principle look principled.

The American mind is being closed in the name of openness -- closed to the idea of reasoned discrimination between ways of living. Bloom says students are taught that all beliefs issue from an abstraction called the ''self,'' a monochrome kaleidoscope, and these beliefs have no validation other than being, by definition, ''self-expression.''

Students are taught that the production of values is an act of will, not of understanding. This is, says Bloom, ''nihilism with a happy ending.'' Understanding is not distributed democratically, but everyone can be willful, just as everyone has a ''self'' to ''express.'' Such teaching induces self-satisfaction that stunts learning: it instills the sense of having nothing to learn from the past or from philosophy.

Bloom writes about music, sex, scholarship, politics. He is passionate and witty. (Bloom, a smoker, says the campaign against cigarettes advances because our relativism does not extend to matters of bodily health, only to matters of the mind.) He has written a ''how to'' book for the few -- not so very few, according to the best-seller list -- who want to know how to be independent.

It is about the arduous task of achieving autonomy, understood not as capricious ''commitments,'' but as governance of oneself in accordance with prescriptive nature. It is living in accordance with philosophy (truth) rather than in subservience to convention, myth, opinion.

Bloom's book, the publishing surprise of the year, is a paradoxical phenomenon. It may be politically portentous. The book's success is evidence against Bloom's severe judgment about the decay of the capacity for reflection about life's large questions. Furthermore, in the 1980s there has been a quickening anxiety about the trajectory of our evolving national character.

Candidates looking ahead to 1988 should look into a bookstore. Bloom's best seller is a timely sign of the high level at which many Americans can be addressed.