I don't know if you've read 'The Mysterious Stranger and the
Critics,' edited by Tuckey, or not - well worth reading, even if much of
it is excerpts from longer works.  Hamlin Hill's biography of Twain ('MT,
God's Fool') covers the last ten years of his life and is an excellent
read for background.

       I'm currently working on a paper on The Mysterious Stranger
manuscripts which may lead into a bachelor's thesis on Twain's later
period myself - score one for synchronicity

Bill Kte'pi