The Hitchiker's Guide to the Galaxy is the best bus ride book ever. I've read it 5 or 6 times on cross country trips.

The idea of the earth as a super computer trying to figure out what the question 42 is the answer to is (of course, run by white lab rats) appeals to me. As does the idea that the current population of the planet was the useless people sent away from another planet...

Man that's a good book!