THE SAGEBRUSH BOHEMIAN: Mark Twain in California by Nigey Lennon (Paragon House: $10.95; 203 pp., illustrated). In this anecdotal biography, Lennon argues that Mark Twain discovered not only his metier but his voice as writer during the years he spent in the West. Using excerpts from his letters, newspaper columns, journals and lectures (material he later reworked in "Roughing It"), she illustrates the development of Twain's distinctive prose style. That style is already evident in a diary note from early 1865, while the author was living in the remote mining site of Angels Camp, after a scandal forced him to quit San Francisco. Noting the to-do over neighbor's new baby, Twain grumbles, "If Mrs. C had given birth to an ornamental cast-iron dog big enough for an embellishment for the State-House steps I don't believe the event would have created more intense interest in the community."