I'd add The Mysterious Stranger, in fact most of Sam Clemens' later fiction which bordered on science fiction.
Yea, Sam got kind of bitter towards the end of his life. Started questioning God and all. Lost his faith. But, some of that stuff is pretty good nonetheless. Pick up a little paperback called "Letters From The Earth". Some good stuff there. Funny too. Sarcastic, cynical. You get the full boat of the later Clemens.