Now that is hard because I might want to take a whole library and could do so if you let it a SD card for the Sony Reader. In the spirit of the question, though, I'd pick the following (the first 3 I already read but would always read again):
- Lord of the Rings by J.R.R. Tolkien (all trilogy in one book )
- Snow Crash by Neal Stephenson (excellent cyberpunk)
- Dune by Frank Herbert (again, I'd consider it a single book)
- Some kind of survival guide
- The World is Flat by Thomas Friedman (everyone says it's an excellent book on globalization)
- The Odyssey by Homer
- Cascading Style Sheets: The Definitive Guide by O'Reilly (because I always wanted to learn more about Web design)