My favorite Twain reference book is EVERYONE'S MARK TWAIN, compiled by
Caroline Thomas Harnsberger (South Brunswick and New York: A. S. Barnes
[c1948 and 1972] 703p.), which consists of excepts from Twain's writings
(with precise references to title, chapter, and page)arranged alphabetically
by subject. A bibliography of works by and about Twain is included. Clara
Clemens wrote the Foreword and concluded by saying:"I cordially
commend...[this reference book] not only to the attention of the lovers of
my father's writings, but to the reader who has not yet become acquainted
with Mark Twain."
Ask a library near you to locate a copy for you to use, or check
http://www.bookfinder.com if you want to purchase a used copy.