It may be slightly out-of-date, but I would strongly recommend a look at
either the first (1958) or second (1966) edition of Philip Foner's book,
Mark Twain, Social Critic.  Foner's book is an example of first-rate
scholarship on the subject of Twain's social consciousness and his writing
throughout his lifetime.  As for Twain's influence on the "social-political
environment of his time and subsequently," it is only necessary to think of
the American historical epochs commonly referrred to as the "Gilded Age" and
"The New Deal."

Martin Zehr
Kansas City, Missouri