Cyril Connolly's literary journalism - a monument to good prose; Enemies of Promise, by Cyril Connolly. New York: Persea Books. 265 pp. $6.95 (paperback). The Selected Essays of Cyril Connolly, edited by Peter Quennell. New York: Persea Books. 307 pp. $17.95.

When Cyril Connolly passed on in 1974 English prose lost one of its guardians. V.S. Pritchett is perhaps the only literary journalist of Connolly's caliber left. And although others - John Updike and Wilfrid Sheed, for example - write about books with great intelligence and enthusiasm, neither has Connolly's elegance or erudition. They don't make them as they used to. And lest we forget how they were made, Persea Books has published ''The Selected Essays of Cyril Connolly,'' as well as reissuing Connolly's 1938 critical autobiography, ''Enemies of Promise.''

Cyril Connolly once wrote that he was ''Alway tired, always bored, always hurt, always hating.'' He was a frustrated novelist, quick to deprecate himself and his profession, journalism. His cranky, petulant voice is unmistakable. But he is more than a brilliant, spoiled schoolboy. Peter Quennell, who edited ''The Selected Essays,'' was Connolly's friend for more than 50 years, and one of the several virtues of this selection is the humor and warmth it reveals in the author. Quennell makes Connolly attractive, not the venomous toad most readers expect.

''Selected Essays'' has three sections: ''Travel,'' ''Life & Literature,'' and ''Satires & Parodies.'' Quennell has chosen an excellent piece on Switzerland, which made me want to book a flight for Zurich, a fine essay on Egypt, and several good ones on Greece. ''Spring Revolution'' is flippant and snobbish, but those are occupational hazards in reading Connolly. He redeems himself with ''The Ant-Lion,'' a haunting piece on Toulouse-Lautrec. ''Farewell to Surrealism'' ends with a moving tribute to Andre Breton, and there is an equally sympathetic account of the Latin poet Propertius in ''The Elegiac Temperament.'' The breadth of Connolly's reading is exceptional, and he wears his classicism, which is profound, remarkably lightly.

There are two pieces on Wilde and three on Joyce, written at different periods, which show the development of Connolly's style and thought (it's too bad Quennell doesn't date the essays, as it would make the development clearer). And there are very funny parodies of Aldous Huxley and Ian Fleming.

Sometimes the quality of the essays seems uneven. Why the silly piece on ring-tailed lemurs when we could have Connolly's essay on Flaubert? Why the long, boring ''Writers and Society,'' which serves only to show what a poor political writer Connolly was? One of Connolly's weaknesses as a critic was his inability to pan a friend, and perhaps Quennell suffers this same generous fault as an editor.

Still, ''Selected Essays'' is a good book. But ''Enemies of Promise'' is a better one. It's a unique blend of literary criticism and autobiography - the point being that books were Connolly's life and not just a hobby or occupation.

''Enemies of Promise'' analyzes the books and authors Connolly grew up with. Just as ''an expert should be able to tell a carpet by one skein of it,'' so Connolly deduces an author's influences, intentions, and importance from one paragraph of his prose. This method has its limitations: It ignores the form of the novel, favoring authors like Hemingway, whose excellence is in his sentences , over authors like Lawrence and Faulkner, who aim at larger effects.

But within these limits, Connolly's treatment works beautifully. His eye is so sharp, his feeling for language so rich, and his reading so close to the text that his interpretations don't seem like opinions at all, but like facts. He never judges without comparison, and he compares authors not just of the same school or historical period, but moves easily from Addison to Keats to Compton MacKenzie.

Connolly distinguishes between two kinds of prose - Mandarin and Vernacular. Mandarin prose is artful; its authors - James, Strachey, Huxley, and Woolf - wish to make ''the written word as unlike as possible to the spoken one.'' The Mandarins suffer when their verbal ability exceeds their thought and they begin to ''spin cocoons of language out of nothing.'' Vernacular prose, on the other hand, is colloquial, the language of conversation. Vernacular authors - Orwell, Isherwood, Hemingway - get into trouble when their prose becomes so bland and impersonal that they are indistinguishable from one another.

In the latter sections of the book Connolly narrates his own history, dispensing advice along the way to future writers, that they may avoid the traps that ensnared him. Don't be too healthy, at least not in England, where ''a healthy writer is communicating with a hostile audience.'' Be sure not to show early promise, like Connolly, for ''Whom the Gods wish to destroy they first call promising.''

Above all, he says, avoid journalism. Happily for us, Connolly ignored his own advice.