Neider--who has made rather a specialty out of ""improving"" on Mark Twain by editing him down to bedside-book readability (cf. A Tramp Abroad, 1977)--now offers a ""frankly idiosyncratic"" selection from the 1917 two-volume edition of Twain's Letters. ""Lively and interesting"" are Neider's chief criteria; he has ""omitted those which are obviously dated, which are trivial. . .""; he has ""also excluded letters in which Clemens indulged his penchant for philosophical determinism. . . . Our country does not need to be taught lessons in pessimism at this time."" This, then, is in no way a balanced self-portrait in letters; nor can Neider's judgment on what is ""trivial"" be taken as absolute--considering his track-record as a freewheeling fellow with the blue pencil. Still, simply as a selection for pleasant browsing, this volume does fairly well. There are letters from Twain's early travels (mostly to mother and sister). There are letters about--and to--his notoriously ne'er-do-well brother Orion. (""You see I have an ineradicable faith in your unsteadfastness."") There's a sarcastic note to the gas company, along with ""unmailed"" mock-replies to annoying correspondents. There's a funny hatchet-job on Sir Walter Scott--plus comments on disarmament, the Czar, Helen Keller, Gentleman Jim Corbett, lynchings, U. S. Grant, the Bible. And, while letters on the deaths of family members supply a bit of emotional ballast, the best bits are almost all in letters to William Dean Howells--re Tom Sawyer, the isolation of ship travel (""I think I foretaste some of the advantages of being dead""), despised European culture, the critics, and Middlemarch. Finally, however, even with Neider's popularizing approach, this is a volume of only erratic entertainment; so Twain enthusiasts will probably prefer to wait for the new Collected Letters, due soon from the University of California Press.