As readers of Thomson's autobiography and A Virgil Thomson Reader know, the composer and critic (now in his 9Os) is a graceful, lean, often droll prose stylist. So this gathering of letters--culled from 25,000 items in Thomson's archives (154 cartoons in his N.Y. apartment)--offers a fair measure of charm, wit, and insight, if little that's emotionally involving or freshly illuminating. From boot-camp, Harvard, and first European travels, the young, slightly arch Virgil writes precociously about food, art, architecture; more personal references are enigmatically terse (""Sex is a bore"") or casually eccentric. (""I took the hair off my legs. . .I am really quite infatuated with my looks as I take morning exercises in front of the mirror."") From Paris in the 1920's and 1930's, Thomson reports on concert-going, socializing, and his work--especially the collaborations with Gertrude Stein. He succinctly defines his esthetic (""play, construction. . .nonsense, magic, and automatic writing""); describes his growing disenchantment with former teacher Nadia Boulanger (""She rooks any work of progress she can get her hands on""); cheerfully offers Aaron Copland a devastating critique of Copland's new book. There's also an atypical bit of introspection in a 1927 reference to friend Mary Butts: ""she is able to uncover certain wells of tenderness and of emotivity in general which have been closed for some years in me."" Most of the letters from the 1940-1954 period are responses--polite, gently amusing--to letters written by readers (usually angry readers) of Thomson's Herald Tribune music reviews. He defends his opinions--""German music has been smelling bad for a long time""--but repeatedly vows that ""there is no bitterness involved"" in his negative reports. And a final, rather disappointing section does include an excerpt from Thomson's collaboration with librettist Jack Larson on the opera Lord Byron. Thomson's ideas about music (his own and others') are far more amply, eloquently displayed in the Reader; the places and people in his life are better encountered in Virgil Thomson. But Thomson enthusiasts will find modest diversion here, along with the frustration of stories only hinted at or half-told.