I use my Amazon wishlists to keep track of what I need from the library. Everything I want to read is on the Private list. If I can't find it at the library, and I think it deserves shelf space, I move it to the Public list.
Usually, I buy the books in the event that I want to re-read them or refer to them. Every once in a while though, our libraries around here will have book sales and there will be something that they’re getting rid of that I might want.