Books and Books and Bucks
If the corner drugstore don't have the book on IPv6 or whatever you're looking for, well, buying on-line's popular. However, if you go to Powell's and shop them against Amazon, you'll find Powell's is more spendy, even though Powell's is Amazon's largest warehouseing & fulfillment partner.
This leads one to ponder if further discounts are to be had by looking further. My favorite tactic has been to go to Froogle and type in the ISBN of the book, and then sort from low to high. Other good sources of prices include ABE Books, Bookpool, Books-A-Million, EveryBookstore and Fatbrain. I'm sure I'm forgetting at least one other good source.
Now, isbn.nu and Booksprice are offering comparison shopping of multiple bookseller locations. Kewl.
Update: A well-read friend adds his recommendation of the AddALL service. Kewler.
Update #2: ThriftBooks, an Amazon reseller, now comes out with their own website, focusing on books at extremely low prices.