Recommended reference books --
2-volume edition:

CRITICAL COMPANION
TO MARK TWAIN
A Literary Reference to His Life and Work
by R. Kent Rasmussen
with Critical Commentary by
John H. Davis and
Alex Feerst
available from amazon.com

This 2-volume edition released
in May 2007 from
Fact on File publishers
updates, expands and replaces previous
editions of
MARK TWAIN A TO Z