I can never get rid of books... I never know when I might want to (or need to in the case of textbooks) read them again! That's why I've filled my bedroom at my parents' house, another bookshelf and some boxes on their porch, my apartment and my share of my office with books.
I never buy new, but if I find a good deal I'll get a big box of 50c-$1 books, like I did Saturday.