Obviously when I spewed an hour ago my mind was on the fiction.  But surely
critics must have discussed  all those essays (esp. late ones) in which
Clemens assaulted various social wrongs--his attacks on the missionaries in
China, King Leopold's Soliloquy, The United States of Lyncherdom, etc., etc.

As with so many Twainian topics, a good book to start with  might be
Howells' My Mark Twain.  Wasn't it Howells (or possibly Paine?) who first
noted Clemens' special blend of historical sense and outrage at injustice?
I can't recall the phrasing, but I think Howells said Clemens could get just
as outraged over a 500-year-old injustice as over the latest  horror he read
about in his morning newspaper.

Mark Coburn