In reply to Ellen Oster's question on the Twain/Twichell
friendship and Twichell's writing concerning Twain --

According to the WorldCat database, Twichell did write at least one
article on Twain that appeared in Harper's magazine, vol. 92, 1896;
p. 815-827.

You might also check Russell Banks' introduction to _A Tramp Abroad_,
Oxford edition.

--Barb