Here is some more info that I received just the other day from Prof. Ed
Folsom:

There's no known response to Whitman's talk by Twain, as far as I know
(Twain's only extended comment on Whitman was a birthday greeting he sent to
the poet on his 70th birthday; it's reprinted in Horace Traubel's *Camden's
Compliment to Walt Whitman*).

Responses to the 1887 Lincoln talk include a review in *The Critic* (no
doubt by Jeannette Gilder, who was at the lecture with her husband, so
there's a Twain connection there) on April 23, 1887.  The New York Times
(April 15, 1887) and New York Evening Sun (April 15, 1887) both ran long
descriptions of the event.  Stuart Merrill was there, and he published a
small pamphlet about the event in the 1920s, parts of which are reprinted in
William E. Barton's *Abraham Lincoln and Walt Whitman* (1928).  Recently,
Daniel Mark Epstein in *Lincoln and Whitman* (2004) has re-told the story
and has quite a bit to say about Twain's attendance at the event, though
most of it is derivative.

Harold K. Bush, Ph.D
Saint Louis University