If you're buying used check all the prices against www.abebooks.com and www.alibris.com. Amazon sellers frequently inflate their prices in my experience, and it's possible to get the same books at substantially lower prices at other online retailers, even though it means paying for shipping. I recently picked up a reference for work that was going for $89 USD on Amazon.com for $25 USD on Abebooks, for example. Weighed that way the $8 shipping was negligible. Of course none of this helps with actually curtailing book buying, but I can quit any time... *twitches*