Amazon.com has a lot of books.

However, there’s quite a few more resources that regularly offer better prices than Amazon:

• half.com
  Apparently this one has been around since 2002. I just heard about it today. Its really good though. I’m disappointed that I didn’t hear about it earlier.
• Thriftbooks.com.

  This one is pretty good. They’ll send you a dog-eared copy of that out-of-print-book for a rock bottom price.

• bookmooch.com.

  This is another one of those next gen web 2.0 projects that seems to be working out ok. I haven’t tried bookmooch, but it looks good and I may try it in the near or distant future.

• College book service
• alibris.com
• abebooks.com
• valorebooks.com
• biblio.com (uk)
• GetTextbooks.com is a great search engine for new and used books
• bookpool.com seems to have some books for discount prices
• International book exchange