This book is for those who understand what REST is, understand the importance of the hypermedia constraint, have maybe read "REST in Practice" (my recommended background reading), have read probably a hundred blog posts about REST... and are still slightly unsure about how to actually do it, when it comes down to the nitty-gritty detail. If you feel like that, join the club.
REST is somewhat abstract, and there's no absolute right way or wrong way. Plus there's many many facets of REST that you have to consider when designing your service.
And that's where this book comes in... in an abstract realm, it gives you wonderfully concrete guidance in the form of Recipes. How to do this. When to do that. Covering the main areas of concern - Uniform Interface, Resources, Representations, URIs, Linking, Atom, Content Negotiation, Queries, Caching, Security and more - there are a decent number of recipes per area that cover all the common questions and concerns that service designers have.
I wanted to know how to evaluate whether Atom Syndication Format would be a good fit for my service... there's a recipe for that. I wanted to know how exactly to implement caching and conditional requests - there are multiple recipes for that.
Plus there are warnings about pitfalls that could save your service... I was going to put crucial information (an API key) in a custom HTTP header, but this book informed why that's a really bad idea.
In short, this book is just essential for all REST service designers and implementors. It currently sits to the side of my monitor, and I consult it many times a day when I'm working on our REST service. You're not going to want to read it from start to finish though: it's purely a reference book... an incredibly useful and practical reference book.
One more thing: it pleases me to see that the wonderful Mike Amundsen contributed to ten of the recipes. Mike's a key thinker in this area, and toilers in the RESTful fields should get to know his work.