We recommend borrowing books from your local library where available, or buying used (and donating it to your local library when you're finished with it).
I've also used http://www.paperbackswap.com with great success. You can list the books that you're willing to give away, and for each one that you send to someone, you get a book credit that allows you to pick a book to receive from someone else.
A great website for finding the best online deals is: http://www.bestwebbuys.com/books/
I've also used http://www.paperbackswap.com with great success. You can list the books that you're willing to give away, and for each one that you send to someone, you get a book credit that allows you to pick a book to receive from someone else.