PHP Web Services is a book in the "animal" series for O'Reilly, my first for this publisher and also my first solo book. It covers the theory behind web services, some specific examples of data formats and service types, then moves on to give good, solid advice on different aspects of writing kick-ass services. The advice is all hard-won, I've been working almost exclusively in this area for a few years now, and I hope that the book has emerged in a way that's not only technically excellent, but also useful, approachable, and applicable to practical projects. I spend a lot more time working with APIs than I do talking or writing about them, which I think is important!
I've included case studies of existing APIs and also included tools and techniques that I'm using when doing this kind of work, particularly around debugging problems and monitoring exactly what's going on. Those skills are important since APIs can be a bit more "black box" than web pages and you may not have the pleasure of owning both ends of the API relationship!
The book is available through O'Reilly (currently on Early Release for digital formats only, the physical version is due May 2013) here:
PHP Web Services on OReilly.com