I enjoy trading books I don't want (duplicates, accidental purchases, gifts I didn't want, etc.) to get books I do want. I use both Paperbackswap and Bookmooch. I've found some pretty rare books from those sites that I really wanted, many of which are hard to find in book stores, or are expensive (for instance I got some manga in the original Japanese). Its also often cheaper than buying the books since you only have to pay the postage to send them out (also paperbackswap gives you the option of buying credits and the price of a credit is often cheaper than buying the book online when you factor in shipping).
I read a lot but have accidentally bought several books that I already had and have been given books I did not want. I also occasionally have to get rid of books to make room for new books (some I got only to read but don't want to keep). My number one entertainment expense is on books. I own several thousand, problem is I don't have room for them or new ones. Throwing away a book is in my opinion a terrible thing to do. I do and have donated books to the library and other organizations.