AbeBooks is actually owned by Amazon now.
Seriously? Wow. Amazon really are dominating that particular market.
From AbeBooks' web site:
AbeBooks Inc. is a subsidiary of Amazon.com, Inc. AbeBooks, an online bookselling pioneer, was acquired in December 2008 and remains a stand-alone operation with headquarters in Victoria, British Columbia, Canada, and a European office in Dusseldorf, Germany.
http://www.abebooks.co.uk/ is pretty good, particularly for difficult to find and second hand books. I've found more and more recently that play.com does almost as well as (and often better than) Amazon in terms of price, although I don't know much about their business tactics.