I read I Been There Before at the time it was published in, I think,
1985.  The premise was that Mark Twain returned to Earth with Halley's
Comet at the time of its reappearance that year.  It is a combination of
fragments of new writings by Twain and reports by others who come into
contact with him as he explores the modern world.  David Carkeet is a
deft and amusing writer who is clearly intimately familiar with Twain's
life and literature as well as with his critical heritage.  In my
opinion, the faux Twain writings ring exactly true--they literally sound
like Twain would have written.  Carkeet has written other good
novels--the book is out of print now, but it's available at over 600
libraries in the U.S.--I don't know if it's available where you are.
It's a book that deserves to be better known, and I think many of the
people on this list would enjoy it a great deal.

Jim Edstrom