From hitherto unpublished writings, here are three more volumes in the ""Mark Twain Papers."" The Blair book, edited by a University of Chicago professor, brings together Twain's fiction and non-fiction commentary on the characters and places he wrote about. Among the narratives included are the uncompleted ""Huck Finn and Tom Sawyer Among the Indians"" and ""Tom Sawyer's Conspiracy."" The Gibson volume represents a feat of literary detection by its editor, a professor at New York University. It contains the three unfinished manuscripts of The Mysterious Stranger, which Twain's literary executor, Albert Bigelow Paine, used to bring out a ""completed"" version of the novel several years after the author's death. Gibson proves that Paine cut, scrambled, and bowdlerized the original texts, with the result that his product did not at all reflect Twain's mood or intent. In the third book, Professor Leary of Columbia has edited Mark Twain's correspondence with a friend and business advisor who was high in the Standard Oil Company hierarchy. The letters present an intimate view of the author's tangled financial life and throw an interesting sidelight on Twain as a critic of the ""Gilded Age.