Not in novel form, but there are actually two Star Trek (the Next
Generation) episodes that involve time travel and Mr. Twain.  I'm not a
trekkie, but my buddy is and he has the "time travel" collection.  They're
"Time's Arrow" (parts 1 and 2).

http://en.wikipedia.org/wiki/Time's_Arrow_(TNG_episode)

It's fun hearing all the suggestions people have,

Michael
Minnesota State University, Mankato