To be honest there really isn't much on p2p in the form of pdb files. Alot of huge pdf's. I didn't find a single pdb book on Kazaa, found a couple on eMule, and a few on Bearshare. Your best bet is binary ebook newsgroups or websites. Memoware.com / Gutenberg Project / Blackmask.com are good places for some quality reading materials.
For some good commercial books check microsoft's reader home page every friday for 3 new books for free throughout the whole summer. I found this just in time, got there the day before they were set to remove Bill Bryson's A Short History of Nearly Everything, a book I was keenly interested in and am currently reading.