From their site: ISBNdb.com's remote access application programming interface (API) is designed to allow other websites and standalone applications use the vast collection of data collected by ISBNdb.com since 2003. As of this writing, in July 2005, the data includes nearly 1,800,000 books almost 3,000,000 library records close to a million subjects hundreds of thousands of author and publisher records parsed out of library data more than 10,000,000 records of actual and historic prices. Using the API you can look up information by keywords, by ISBN, by authors or publishers, etc. In most situations the API is fast enough to be used in interactive applications.
The following is a list of historical changes that were made to this asset. Typically, once an API is published on ProgrammableWeb.com, the data we keep about that asset gets updated over the course of its life. For example, the endpoint or home page of the documentation might change. As a means of enforcing the integrity of our data, every time a non-trivial change is made to the data we keep about an asset, we require the entry of some changelog text before that change can be saved. This way, we have a great audit trail of all activity surrounding the asset. For each asset, the changelogs begins with the moment an asset is approved for publication on ProgrammableWeb. If you spot an error in our data and believe that a record needs to be updated in some way, the ProgrammableWeb team would very much like to hear from you at firstname.lastname@example.org