The Amazon Historical Pricing web service gives developers programmatic access to over three years of actual sales data for books, music, videos, and DVDs (as sold by third-party sellers on Amazon.com). Sellers can use Amazon Historical Pricing to make informed decisions on pricing and purchasing.
Unfortunately it seems to be limited to getting 10 items at a time:
* Returns pricing information for up to ten items per request
Which will limit the potential for this to be used by book historians as a kind of barometer for cultural or economic change.