I'm not sure I can be of much help in recommending criticism, but I, too,
feel that MT's later work, especially _The Mysterious Stranger_
manuscripts and the _Letters from the Earth_ collection, are some of his
strongest satirical pieces.  I'm sure there are passages that lean
toward burlesque, and as has been observed, much of the quality of the
writing in the later work suffers from Twain's career as an
orator fond of one-liners, but never would I have chosen the word
"weak."

Gibson's comments in the intro to his version of _MS_ might prove
useful, as well as DeVoto's comments in _LFTE_.  My area of interest is
19th century science fiction, and Twain's later work figures heavily in
this area, so some of the Science Fiction journals (e.g.,
Science-Fiction Studies) have done some interesting critical work on the
later stuff.  You might try _The Science Fiction of Mark Twain_ by Ketterer.

Gibson, unfortunately, did not provide a bibliography; I'm not sure about
Devoto (my copy is MIA).  I don't know about the more recent
criticism.

Jeri Zulli