Alan Gribben's *Mark Twain's Library: A Reconstruction *is an amazing work
that would best answer this... from my readings of it I would suggest that
the Bible would be by far the most influencial.  *Don Quixote *is
anotherwhich, according to Gribben, he called one of his "beau ideals
of fine
writing", and once loaned to Susan Crane (his sister in-law) saying "I hold
her strictly responsible for it.  And she might as well abuse Livy as abuse
that book", and wrote that Cervantes had the "best of opportunities" for
writing a masterpiece—"solitary imprisonment, by compulsion".

Also, despite his dislike for Austen, Poe, and Cooper... he seemed to have a
good supply of all of their works.  It would be an interesting thesis to
write about why Twain surrounded himself with works by authors he despised
(and perhaps how the differences he saw between himself and them shaped who
he ended up being).
According to Gribben he also wrote about Stevenson's *Treasure Island* and *The
Strange Case of Dr. Jekyll and Mr. Hyde*, "God, so atrocious in the Old
Testament, so attractive in the New--the Jekyll & Hyde of sacred
fiction/romance.  Stevenson plagiarized it?"

It will be interesting to see what other's say.

Michael