- Half Price Books - Sign up on their mailing list to receive coupons.
- Thrift stores - I've found independent stores a better source (as opposed to Goodwill, etc).
- Paperbackswap.com - For every book that you ship to a member, you receive a credit. You can use that credit to order a book from another member. You only pay shipping. The secret (shh!) is to mail very light books in order to build up your credits inexpensively. Thin children books are perfect for this if you have them around your house. Be sure to add books to your wish list. When they are available you will receive an email with the option to order it.
Other good stuff:LibraryThing An online catalog of the books you own. To add books, simply enter the title or ISBN in the search box. A list of results will appear. Click on the title you want to add. You can also add a book manually.
Bookfinder Use to find and compare prices on new and used books.
What are your favorite book resources?