Recently the nice folks at Addison Wesley sent me a review copy of "The Developer's Guide to Social Programming" by Mark Hawker (you can find it here on amazon). This is right up my street as I'm doing so much with with APIs, social and otherwise, at the moment and the book covers things I'm using such as OAuth and google's services.
All in all I think this is a good addition to the bookshelf for anyone adding social features to websites. The exact information will soon be outdated, but it's currently accurate enough to be useful. The prose is clear and easy to follow; it's a slim book but if this is the topic you want to learn about, then this is the book to get