It may be slightly out-of-date, but I would strongly recommend a look at either the first (1958) or second (1966) edition of Philip Foner's book, Mark Twain, Social Critic.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