THE SAGEBRUSH BOHEMIAN: Mark Twain in California by Nigey Lennon (Paragon House: $19.95; 191 pp. ). Nigey Lennon begins this study with a good premise: that Mark Twain's previous biographers and critics have slighted the writer's Western years, which included newspaper work for the Virginia City Territorial Enterprise and travel writing for the Alta California, among others. Those writings have generally been considered no more than warm-ups for later, greater novels, and, truth be told, they are apprentice work--with the important caveat that Twain's youthful journalism was formative in that it allowed him to find his role as a biting social critic. The East-slighting-West formula underlying "The Sagebrush Bohemian"--the sobriquet was given Twain by a rival reporter who intended to cut--is a familiar one, but in this case largely true . . . and interesting, because Twain himself, once he had become a national literary figure based in New England, downplayed his years in California and Nevada. Lennon's writing style is at times annoyingly and self-consciously clever, but she amply demonstrates that Twain's approach to his work, if not always his subject matter, was much indebted to his experiences in the fluid, boom-and-bust, rough-and-tumble frontier society.