Some additional Twichell resources:

Leah A. Strong, _Joseph Hopkins Twichell: Mark Twain's Friend and
Pastor_ (Athens: University of Georgia Press, 1966).

Mark Twain -- Joseph H. Twichell Correspondence, American Literature
Collections, Beinecke Library, Yale University.  I don't remember if
Twichell's letters are in this collection but it is a good source for
Twain's letters to Twichell.  Albert B. Paine edited a number of the
letters (often silently) before including them in his 1917 edition of
_Mark Twain's Letters_.

Kenneth R. Andrews, _Nook Farm: Mark Twain's Hartford Circle_
(Cambridge, MA: Harvard University Press, 1950).

The article by Twichell that Barbara Schmidt mentioned is online in
the Cornell University section of the Making of America project:

Joseph H. Twichell, "Mark Twain"
http://moa.cit.cornell.edu/dienst/moabrowse/
MOA-JOURNALS2:HARP-0092/827/1:GIF89:100

Jim Zwick