I buy both new books and used books. It really depends actually. Some of the best books are really old and rare, you can hardly find them on bookstores that sell brand new ones. The best books I've got now and mostly my favorites came from vintage and thrift bookshops.
I do love new books, but I would also buy used. They are just as good, so if I found them at a good enough discount I would buy. Perhaps not so much for a leisure book but certainly for textbooks etc.
