These are more specialized reference tools but might be good to add
to the bibliographies list:

Louis J. Budd, _A Listing of and Selections from Newspaper and
Magazine Interviews with Samuel L. Clemens, 1874-1910_, published as
a special issue of _American Literary Realism_ 10:1 (Winter 1977),
entire.

Louis J. Budd, "Listing of and Selections from Newspaper
and Magazine Interviews with Samuel L. Clemens: A Supplement."
_American Literary Realism_ 28:3 (Spring 1996): 63-90.

Machlis, Paul. _Union Catalog of Clemens Letters_  (Berkeley:
University of California Press, 1986).

Machlis, Paul. _Union Catalog of Letters to Clemens_ (Berkeley:
University of California Press, 1992).

These are also online at Union Catalogs of Clemens Correspondence
http://bob.ucsc.edu/library/mtp/index.html
Only the letters from Clemens (and family) are included in the online
database so far.

A combined index and full references for the Addenda and supplements
to Tenney's _Mark Twain: A Reference Guide_ that appeared in
_American Literary Realism_ (1977-1983) and the _Mark Twain Circular_
(1987-1988) is available at the Mark Twain Forum's web site:

http://web.mit.edu/linguistics/www/forum/mtcindex.html

This document also includes volume-by-volume indices of later issues
of the _Mark Twain Circular_ that can be easily searched online.

Jim Zwick
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http://www.accinet.net/~fjzwick/
http://marktwain.miningco.com/