Selected Letters of Virgil Thomson Edited by Tim Page and Vanessa Weeks Page 413 pages. Summit Books. $24.95.

It is tempting to begin a review of Virgil Thomson's letters with broad pronouncements about distinguished careers and profound effects on American music. Then one remembers Mr. Thomson's quiet wit and hastens to cool it. Virgil Thomson, the composer, and Virgil Thomson, the music critic and letter writer, succeed at exactly the same things -cutting through the oratory, theorizing and pontification of the modern music world, staying loyal to one's roots, then speaking clearly yet gracefully about them. In his long life, Mr. Thomson has waged a subtle yet merciless campaign against pomposity, and it is still wise to stay out of the way.

The roots in question are in Kansas City, Mo., and all the years of study and work in Paris between two world wars only sharpened his affection for his origins. Mr. Thomson grew up liking the Protestant church (where he played the organ) and its hymns. He also liked black American music for its sophisticated movement and clear speech. The music he heard in the streets, parlors and pews of his childhood all found its way into his compositions. It never left.

Mr. Thomson went to Harvard and then to Paris (from 1925 to 1940), where he studied with the great guru to American musicians, Nadia Boulanger. In his letters, Mr. Thomson ignores all the reverent attempts at canonization and states clearly what she was good at and what she wasn't. His friends and collaborators were Gertrude Stein, Alice B. Toklas, Sylvia Beach, James Joyce, Janet Flanner - all the familiar figures of the so-called Lost Generation. Whether Erik Satie in particular and French music in general shaped his art or whether France just suited what was already inside him, Mr. Thomson became a sort of guidepost for American composers who, tired of following German models, started to pursue French ones.

With the war, Mr. Thomson returned to New York in 1940, where he was music critic at The New York Herald Tribune for 14 years and where he still lives, aged 91. He seems to have answered almost all the letters from his readers, no matter how irate. These responses are dignified, genuinely sympathetic, unapologetic and often very funny. They also form the strong center of this collection by Tim Page, chief music critic for Newsday, and his wife, Vanessa Weeks Page.

To a standee disgruntled by a Mozart performance, for example, Mr. Thomson writes: ''If I had stood through 'Don Giovanni' I am sure I, too, should have found it sour. As it was, I had a good nap. If Miss S ---had committed grave misdemeanors about pitch, I am sure I should have waked up. At musical performances I sleep lightly, and only so long as nothing in any way abnormal, for good or ill, takes place on the stage.'' To a perturbed Mrs. August Belmont, founder of the Metropolitan Opera Guild, he confides: ''Perhaps criticism is useless. Certainly it is often inefficient. But it is the only antidote we have to paid publicity.''

The early and later correspondence is also entertaining and provides us fragments of Mr. Thomson's daily life, friends, work habits, cooking skills, tastes and his unassuming courage in the face of encroaching deafness. Yet these entries are more about an interesting life than the process of a musical mind. To read the letters between Stravinsky, Charles-Ferdinand Ramuz and Rene Auberjonois concerning ''L'Histoire du Soldat'' (''Stravinsky: Selected Correspondence. Vol III,'' edited by Robert Craft and published by Knopf), is to follow with fascination the practical day-to-day decisions in the building of a masterpiece. Mr. Thomson's letters occasionally descend into detail -especially about the ''Four Saints'' productions - but a coherent look at how and why Mr. Thomson works at music just does not want to emerge from this correspondence.

The absence, I think, is not because of his editors but their subject matter. Mr. Thomson doesn't really show us much about himself. His wit observes the world but at the same time deflects our attention from the observer. Mr. Thomson's deeper sensations - doubts, fears, loves, angers, all the things that fuel a creative mind - he keeps to himself. Boulanger once said exactly that: ''In spite of the fact that I am acquainted with you as a talented musician, a charming person, and an incredibly intelligent young man, I have the feeling of not knowing you at all inside.'' This is a quote recorded with accustomed candor by Mr. Thomson in one letter. And of course, he is entitled to his privacy.

One wonders which will end up being most valuable - Mr. Thomson's music, his writing or his apparently endearing self. I, for one, find the music fun to think about but not that interesting to listen to. Perhaps that is also why I think the Stravinsky letters more fulfilling than Mr. Thomson's. ''L'Histoire,'' in other words, has an enduring substance that ''Four Saints,'' for all its charm, does not.

The back-cover blurbs for this collection credit Mr. Thomson with ''as profound a vision of American culture as anyone has yet achieved,'' and also as being ''the century's most bounteous dispenser of musical champagne.'' We all like to be loved, but I suspect Mr. Thomson might be suspicious of such inflationary language.

His main virtue is less grand -simplicity without simplemindedness. Americans at their best all seem to have it - from the oldtime he-men you used to find in the movies to the prim and sedate likes of Virgil Thomson. Spencer Tracy, who played himself in every role, was not deep, but he looked us quietly in the eye, and our pretensions just melted away. Good Americans refresh us not with profundity but with directness. Virgil Thomson is one of them.

Photo of Virgil Thomson