Two other sites to consider:

computerbookstore.com often has sales of 41% off titles by a specific publisher.

abebooks.com is a portal site for many used book stores and contains a search engine that locates available stores and lets you limit searches by specific countries as well as convert prices to your local currency. I've used this to find out-of-print titles and used versions of many technical books. The tech books are often at least 50% off retail. For example, a search performed a few minutes ago listed a copy of Learning Perl, 2nd Ed. for $8.00.

hope this helps...

-- footpad