There are passages that would allow Wills to make this
judgment. In the Innocents Abroad there are some sharply
contemptuous references to native Americans ("The James Fenimore
Cooper tribe"). In the Gilded Age, there is a little section
about Irish women who have changed their names to sound French.
Both of these were somewhat early in his career, and they do not
make a valid reflection on the character of the more mature Twain
who was more or less down on the entire human race.
I think Wills made something of a cheap shot here. It is
always problematical to look at somebody's race ethics through
the magnifying glass of late 20th century sensibilities. I
remember reading McCullough's book on Truman recently. Some of
the letters that a young Truman wrote would probably make David
Dukes blush, but this was not a good predictor of his later
behavior as President. Both men were a product of their times
and place, and both grew a lot.