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.
I’m so short of shelf space, and OUT of walls to put more shelves against, that I have to be very discriminating about what I actually acquire. If it’s popular and in print, I figure the library will have it for a long time, and Lord knows they’ve got more storage than I do.