Okay, so I'm gonna give away how much of a nerd I am here... but Isaac Asimov wrote a great book on the paradox of altering time, "End of Eternity." Pick up a copy right away, because I understand that Hollywood is going to butcher it into a film soon, like it did with "I, Robot." Another good one is Ray Bradbury's "A Sound of Thunder," which was already been ruined by a small movie studio a few years ago.