Twain's relationship with Carnegie deserves to be studied more than it
has been.  Carnegie, along with quite a few others, seems to fall into
the vague category of "friends," and not much research (that I'm aware
of, anyway) has gone beyond that to look at the nature of the
relationship.  There is no doubt that Twain and Carnegie were in at
least roughly the same New York social circle during the 1900s, but
so were quite a few other people, and a study of Twain's position
within that larger group of early 20th century New York society would be
quite interesting.

Besides his involvements with the anti-imperialist and peace
movements, Carnegie also contributed financially to the Southern
Educational Board founded by Robert C. Ogden in 1901 to improve race
relations in the South by providing educational opportunities for
African Americans, and in January 1904 (while Twain was in Europe) he
spoke at a conference held in Carnegie Hall to advance unity among
African American leaders (see Ralph Luker, _The Social Gospel in Black
and White_).  We know that Twain was also very interested in race
relations during the same period, but I don't know that it was
something Twain and Carnegie ever discussed.

_Autobiography of Andrew Carnegie_ includes some information about
their relationship, and Carnegie also wrote  a memorial tribute to Twain
that can be found in the _North American Review_ 191 (June 1910):
827-828.

Some of both Carnegie's and Twain's anti-imperialist writings are
available online through my Anti-Imperialism in the United States,
1898-1935, web site at

http://web.syr.edu/~fjzwick/ail98-35.html

Two of Carnegie's essays are listed on the Essays, Speeches and
Pamphlets contents page and a large section on Twain can be found
through the Literature contents page.  My essay there on "Mark
Twain, William Dean Howells and the Anti-Imperialist League,
1899-1920" might be useful because it discusses a split within the
League in 1904-1905 in which Twain, Howells and Carnegie all took
different positions.  Carnegie apparently paid the printing costs for
the Anti-Imperialist League of New York's pamphlet version of "To the
Person Sitting in Darkness" but that, in itself, shouldn't be interpreted
as indicating a special personal friendship.  Carnegie was one of the
League's primary financial supporters and paid for numerous
other publications.

There are also some _New York Times_ articles about both available
online through Barbara Schmidt's Mark Twain in the New York Times
page at

http://www.tarleton.edu/activities/pages/facultypages/schmidt/nytindex.html

Jim Zwick