Paper books, with very few exceptions. If I need to reference a textbook or some other technical book for one specific thing, I'll dig up an e-book for it to read over the relevant passage(s), but for virtually everything else I'll invest in a physical copy.
I do that mainly because I get distracted too easily when on the computer to read PDFs of any significant length. For that matter, my e-reader isn't backlit, and I don't currently have the spare funds lying around to invest in a better one—not that I'd want to drop a bunch of money on one, anyhow, since like a lot of the other people here I enjoy the physical sensation of holding the book in my hands when reading it.
Interestingly enough, the physiological engagement that you get from the paper against your skin and the smell of the pages actually helps you to remember the book in greater detail than you would were you to read the same book with an e-reader or on a computer. This is because your brain is better able to encode memories when the associated experiences activate more of your senses.