Fine prose rediscovered; A Language Not To Be Betrayed: Selected Prose, by Edward Thomas. Edited and with an Introduction by Edna Longley. New York: Persea Books. $22.50.

Edward Thomas's reputation as a poet has grown steadily in America since Norton & Co. published his ''Collected Poems'' in 1974. Since the appearance of Lawrence Thompson's monumental life of Robert Frost, many readers have sought out the poems of this retiring Englishman whom Frost held in such great esteem. In the last few years poets and critics on both sides of the Atlantic have come to regard him, along with Wilfred Owen, as one of the best rising British poets on the eve of World War I. His writing career poet was cut short, however, by his death in the Battle of Arras in 1917.

But Edward Thomas was serious about his own verse only during the last five years of his life. His major endeavor over several decades was prose, book reviews mostly, and books on nature and the British countryside. He also wrote book-length critical appreciations of English writers. Between 1901 and his death, Thomas reviewed about 1,200 books.

Now Edna Longley has selected some of the best pieces from this mountain of prose for a volume called ''A Language Not To Be Betrayed,'' and this new book cannot help but advance Thomas's reputation even further.

These short selections provide American readers with a window through time, back to a simpler age when Thomas Hardy, the best contemporary English novelist, was just coming into his own as a poet; when William Butler Yeats had yet to compose his best work and was submitting to the criticism of an annoying young American named Ezra Pound. It was a time when an unknown American named Robert Frost, whose poems had been unwelcome in his own country, appeared on the British literary scene with the most radical new theory of composition since Wordsworth.

All these great writers come under Thomas's thoughtful scrutiny and demand appreciation. It seems odd at first that a man of his wide knowledge of literature should have paid much attention to such minor lights as Lascelles Abercrombie and other ''Georgians,'' whose reputations have long since flickered out. Yet when Thomas was writing, their poetic careers were still open to development, and he was generous enough as a critic to encourage real talent wherever he found it.

Edward Thomas also had an eye for greatness and recognized almost immediately the place of D. H. Lawrence among the great modern poets. His knowledge of literature in English seems to have been as comprehensive as that of his slightly younger contemporary, T. S. Eliot, and his critical acuity was probably as great. Unlike Eliot, though, Thomas did not see himself as a crucial master poet in a tradition of master poets who dominate English verse. Rather, he seems to have regarded himself as one among many serious practitioners of verse and prose who are too busy struggling with words to worry about their own places in the grand scheme.

He might have enjoyed teatime with the Welsh tramp poet W. H. Davies, whose best work he admires, quite as much as with the great American. Whereas Eliot's wisdom concerning the merits and reputations of fellow poets was delivered to an adoring public in carefully polished pronouncements, these contributions of Edward Thomas are scattered among the hundreds of reviews he wrote to feed his wife and children.

Edna Longley has also included selections from Thomas's autobiographical work. There is a wonderful smidgen from ''Feminine Influence on the Poets''; every poet who is serious about writing English ought to read it, but it has been almost impossible to get hold of in the United States. Why did Ms. Longley include so little of this wonderful book, which tells us more about the creative process than many tomes of clinical speculation?

Perhaps her volume will lead to the reissue of some of his better critical and nature writing. In any event, it is a book that will delight not only poets and scholars, but all serious lovers of English literature.