Also not in novel form, but Twain was featured in an episode of Touched by
an Angel in 1997 ( in
which the angelic regulars on the show encounter Twain in his late years
and try to convert him from his pessimism.  Unsuccessfully, to the
distress of the angels.  Who turn sour and get discouraged.  They have to
go on meds and get help.  That's the next episode.  I don't think it was
ever aired.

Jean is in the episode, too, and Kate Leary.

Terry Oggel