I voted for most but not all books, but I was a little torn between that and the next one down. I buy all the books primarily focused on rules and monsters. The settings and adventures are where I only buy the ones that I have direct interest in.
I feel like I currently own enough material than I could use for the foreseeable future (with some third party adventures in there as well) but I don't think my buying habits will change anytime soon. Even if I won't use something in a game for a long time after I buy it just reading through the material is enjoyable for me.