iDreamBooks, book review aggregator, deems itself the "rottentomatoes.com for books." iDreamBooks searches reviews across media outlets and compiles the data to a single, comprehensive repository of review. iDreamBooks offers developers access to the reviews through the iDreamBooks API.
It's always exciting to see those outside the web industry using an API to improve their sites. NY Times First Look blog has a great example of what Dallas Public Library has done with the Best Sellers API.