If the Scouts ruled America ...; The Boy Scout Handbook and Other Observations, by Paul Fussell. New York: Oxford University Press. 284 pp. $15.95.

It's one of the peculiar misfortunes of the late 20th century that those who seek to instruct us too often write in prose as impenetrable and resistant to our approaches as a can of anchovies. Paul Fussell is a writer whose works offer no such discouragement. He writes with a clarity and a sense of humor that effortlessly illumine the serious and moral concerns of his books. This latest book continues the fine record he has established as a cultural and literary critic.

As a collection of essays it ranges far: an examination of the latest Boy Scout Handbook, a discussion of whether Graham Greene can indeed write, book reviews, notes on travel, commentary, and observations on Americana and on World War II.

It is in the title essay that Fussell, writing always with a light touch, suggests what is absent from so much contemporary life. Fussell acknowledges, of course, that such a handbook contains much practical advice: on how to start a fire, etc. But the overwhelming impulse, beyond such estimable common sense, is goodness, and the happiness which is associated with virtue. ''Indeed, this handbook is among the very few remaining repositories of something like classical ethics, deriving from Aristotle and Cicero,'' he writes.

There is no mistaking the serious intent in his conclusion: ''Actually there is hardly a better gauge for measuring the gross official misbehaviour of the seventies than the ethics enshrined in this handbook. From its explicit ethics you can infer such propositions as 'A scout does not tap his acquaintances' telephones,' or 'A scout does not bomb and invade a neutral country, and then lie about it.' '' It's a dazzling piece of social commentary and prescription, precisely because the source seems so unlikely, so beneath conventional intellectual contempt, let alone consideration.

Like Nancy Mitford's tongue-in-cheek divisions of U and non-U, Fussell's rankings of class are not to be taken too seriously, though his observations are acute. He includes a questionnaire that asks, for instance, to which class we would assign ''a young woman trust officer in a large New York bank, who loves to watch Channel 13 WNET, and likes to be taken out to restaurants said to serve 'gourmet' food.'' (The answer is ''Middle with hopeless fantasies about being Upper-Middle.'')

Fussell asks after all this apparently inconsequential fun whether class does indeed involve something more than what we watch on television and when we eat dinner. Perhaps the aspiration we all have to rise in the system is indeed more a tribute, if unperceived, to independence, freedom, and grace; more a question of ethics and aesthetics than some arbitrary list of do's and don'ts.

It is this introduction of a profound and far-reaching concern into a discussion sometimes superficially frivolous that distinguishes Fussell as a writer. He sees and makes the connections that reveal the wider implications of subjects as diverse and unlikely as a collection of Life photographs, a biography of Boswell and a review of literary atlases. There are few contemporaries who can equal his engaging manner, his readiness with the appropriate quote, and his clear thinking.