"Mark Twain was now regarded by many as the foremost American author. Certainly he was the most widely known. As a national feature he rivaled Niagara Falls. No civilized spot on earth that his name had not reached"

Another letter addressed: "Mark Twain, The Devil Knows Where"
also reached him, and he answered, “He did, too.”

For all of Mark Twain's books, Bookyards has them at http://www.bookyards.com/biography.html?author_id=36&author_name=Twain%2C%20Mark