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Is there an API that can find books from multiple sellers by say, ISBN? It would be nice to easily find the cheapest book from various sellers.

Similar but related, I use this: http://www.cheapriver.com/

It compares prices amongst Amazon: different country sites price things differently, the site above compares them with postage. It's often cheaper to buy from a different country.

Not sure if it's exactly what you're looking for, but I use a Firefox Add-on called Invisible Hand.

http://www.getinvisiblehand.com/

When you visit a product page on Amazon or other retailers, a notification bar will show you the lowest price for that product.

I just buy direct from O'Reilly or Pragmatic Programmers' websites. The books are DRM-free and they come with free updates.

Or better yet, for the price of just a few books you can have access to a really decent library at acm.org or computer.org

Get your books more cheaply – possibly