The mystery that was referred to by Larry Howe is by Julie Smith and is
called, "Huckleberry Fiend". It is an enjoyable book and gets most of the
Twain stuff right as the plot romps around the SF Bay Area and Virginia
City.
Of course, the finding of the first half of the manuscript seriously dates
it
now.

Another less enjoyable mystery (pre-dating the Peter Heck series currently
underway) was "The Mark Twain Murders" by Edith Skom. It had little about
Twain and even less to recommend it.

Dennis Kelly