This is a good book on the principles of RESTful web services and a useful reminder of the principles of REST itself.
Clearly written with lots of examples. The authors are clearly passionate about REST and RESTful services and explain their viewpoint well.
However, at times, the passion spills over into polemics, which can distract.
For me, the worst aspect of the book is that the bulk of its examples are written in Ruby. I'm not very familiar with Ruby or Ruby on Rails - and Ruby syntax is hard to grasp for the uninitiated (ie me!). It also works some examples that depend on particular Ruby libraries that don't have counterparts in other languages.
It is particularly disappointing that there are not more examples in JavaScript, addressing the substantial Ajax community - Chapter 11 deals with "Ajax Applications as REST Clients" and covers useful ground, but it would be better to see more.