PHP Web Services 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, and I've aimed to share it in a way that's useful, approachable, and applicable to practical projects. I've included the tools and techniques that I'm using when doing this kind of work, particularly around debugging problems and monitoring exactly what's going on.
The book is available through O'Reilly here:
PHP Web Services on OReilly.com