Congrats to Paul Reinheimer whose book on Web APIs with PHP is now available. I haven't read it yet, but I know Paul has been working on it for well over a year, and it has already received a positive review from Nathan Smith:
If you are looking for a good book describing how to make use of the various web services out there, look no further. Professional Web APIs with PHP is a compliation of several helpful tutorials covering the more popular web application programming interfaces (API) available.
My copy is in the mail.