I have read the three volumes of autobiographies; would I find anything new in "Mark Twain's Notebooks" edited by Carlo de Vito?
BTW, that book suffers from the curse of the seldom-reviewed on amazon; it has an average rating of 3 starts, but there are only two reviews: one reviewer gave it the highest (5) and the other the lowest (1). This is probably a case where statistics lie, or at best stretch, things.
 - B. Clay Shannon