Good Omens by Neil Gaiman and Terry Pratchett. Quite possibly one of my favorite books, I've recommended it to just about everyone I have met that has an interest in reading. It is a non-serious comedy, so if you want a good laugh, freaking read it.

Now I have all these memories of awesome parts of the book swimming though my head. Too tempting not to share, like when one of the four horsemen of the apocalypse, Pestilence, retires in 1936 muttering something about penicillin. Poor guy.