Too many. I'm not completely sure about how many of my 3.x books are actually from the 3.5 era. It's clear that I bought less and less WotC 3.5 books in the late years, because I didn't really like them anymore. On the other hand, they published more, so the 3.5 years have some kind of headstart. Plus, there are the late Dungeon and Dragon magazines. I have lots of 3rd party books, though.