Sadly, it looks like bookpool is no more.  

http://www.bookpool.com

From their website, 
"Bookpool LLC offers you great technical book prices, excellent service, and fast delivery. All our books are brand new and purchased directly from the publisher. By ordering large volumes of books that are in high demand among Internet users, Bookpool LLC receives sizeable discounts that it passes on to the worldwide Internet community."

Great if you need to purchase for your entire department/development staff.

I find it a good place to find technical books, in general.  For example, they have ExtremeProgrammingExplained (http://www.bookpool.com/.x/n6x7qzrx8n/sm/0201616416) cheaper than Amazon.

Contrary to recent rumors, Bookpool has no affiliation with amazon.

(Disclaimer - Last edited by a bookpool employee to fix some inaccuracies)