This weekend I had the opportunity to read RESTful PHP, by Samisa Abeysinghe, published by Packt Publishing. The book is short, about 200 pages, but full of great information about what REST is, how it is used, how it is supposed to be used and how to use it with PHP.
The book assumes you have a working knowledge of PHP, and how to install extensions (or use existing ones). The concepts are clear and concise. Samisa is direct, but explains the reasons for why code is the way it is, or explains what can be done different.
The books runs through many examples using Yahoo! and Amazon APIs. Most of the book could be called a manual for use of CURL and SimpleXML, as well as some DOM work. Although you will not need to know any of those things before picking up this book.
In a short seven chapters Samisa explains what rest is, who uses it, why it is used, and what you can do with it. Samisa walks you through consuming (or using) REST APIs and also how to setup your own APIs using the REST style and architecture.
I would recommend this book to any beginning and middle tiers PHP developer as it is a quick reference to REST and APIs that you know you already want to be working with.