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.
The Library makes it easy to find the most popular books on the website with a special best-sellers section. From the list you can click to search by ISBN, one of several values returned by the Times API.
The implementation is, perhaps, a little simplistic. However, the User Experience of the list isn't the part we should be admiring. It's that libraries are embracing open APIs and creating useful information access applications on top of them. It's a big enough trend that there's a good new book on exactly that topic from Nicole C. Engard entitled Library Mashups.
Library funding is dwindling. Perhaps getting back to the basics of what a library provides--information--is the key. Like newspapers, libraries cut through the clutter and find the knowledge we need. Connecting to APIs with data that gets us there is one new way to do just that.