1. Slaughterhouse 5 - Kurt Vonnegut (Fiction)
2. The Prince - Nicolo Machiavelli (Political Science)
3. The Republic - Plato (Philosophy)
4. The Hitchhiker's Guide to the Galaxy - Douglas Adams (Science Fiction)
5. Hamlet - Shakespeare (Literature)
Adams and Vonnegut are available for purchase, the rest are PD. I know these are all pretty obvious, but any list worth it's salt will have these on it.