This is the best site that I have ever found for buying books that are considered overstocks or "remainders". I probably buy at least half of my books from this site. Give it a try.

Hamiltonbook.com

Thanks Observer! That site is GREAT! I have already bookmarked about 10 books I want to buy as Christmas presents.

Also try http://www.half.com/

Half.com is good and is run by Ebay. You can also try Ebay because you can usually find an auction that is selling the same book cheaper. My girlfriend loves Mary Higgins Clark books and we are getting hard-cover books for $3 a piece on Ebay when the new paperbacks are $8 a piece on Amazon. We found someone selling seven of the books that she wanted and we emailed them and asked if they could combine the shipping to save on costs and they are going to ship all seven books to us for $7.

And, if I may add...
if you're looking for old, out of print or collectable books:
www.abebooks.com

Well I just got my books from Hamilton that I ordered, and they are great! They are in great shape (I did not opt for the shopworn versions, however). I can't believe i got a hardcover CIA world Factbook 2000 for only $7.95. I saw a 2002 factbook selling on amazon for $95.00. I'm sure there are some things out of date, but a great reference book all in all!