Well, the most recent work on this period that challenges the old
consensus is in Karen Lystra's new book with UC Press.  Lystra,
unfortunately, does not give a good bibliography of the books and
articles that challenge that consensus, but I think she's right to point
out that Clemens was not just a writer and that as a public figure in
other forums, his moods were often jubilant.  He comments on that
himself in his autobiographical dictations (See my last chap in
_Sentimental Twain_).  Much of his writing, not meant for public
consumption, was cathartic, a reflection of his anger and pain, but not
reflective of his entire life.  And even some of that writing is quite
funny in a very playful way.  I think of "3,000 Years among the
Microbes" and "The Secret History of Eddypus."

Moreoever, he had his dark moods throughout his life, so it's hard for
me to see that the late period was so radically different.  _Yankee_ is
apocalyptic; _Huck_ is not much better.  He considered suicide at least
once in the 1860s.

But, in light of my previous post, I'm not sure I'd trust my memory or
judgment if I were you.

Gregg