The RSpec Book is a 400 page book by David Chelimsky and a cadre of BDD (Behavior Driven Development) experts. Its aim is to teach you all about RSpec (RSpec 2.0 - specifically!) and BDD from the ground up and it promises to "help you write better code, write better tests, and delver better software to your users."
Robert C. Martin (a.k.a. Uncle Bob) kicks off the book with a foreword that warns us of what's to come. He says that the book is a trap and isn't really about RSpec. I won't spoil the whole surprise of his delivery but his general point is that the book is focused on teaching you software craftsmanship using BDD (and testing in general) as the framework for putting together well-crafted software. This point is significant because The RSpec Book focuses on the concepts of BDD just as much as it does on the technicalities of RSpec itself.
The book starts with an extensive Getting Started section headed by a quick chapter summarizing RSpec and Cucumber before moving on to a suite of walkthrough-style chapters dedicated to building a 'code breaker' game. Acceptance Test-Driven Planning is used which essentially means the acceptance tests are written first in the form of Cucumber features so for two chapters you don't get to see any RSpec at all. Once RSpec comes into the mix, though, things move quickly and mocks (doubles) and stubs are introduced quickly. The 'code breaker' game work then continues for a couple of chapters with a brief detour into refactoring.
The second section of the book - Behavior Driven Development - is made up of two code-free chapters that look at BDD from a higher level. A lot of this portion is quite opinionated but if you want to get an overall feel for the BDD process and how different concepts interlock with it, it's a great primer.
The third section of the book - RSpec - proved to be the real "meat" for me. There are several chapters digging solely into the ins and outs of RSpec 2.0 itself. You learn how to use RSpec from the basics up, working through matchers, best practices, mocks, macros, custom formatters, custom matchers, and how the RSpec toolkit can integrate with other tools (such as TextMate). You basically get a 102 page guide to RSpec 2.0 here and that might be worth the price of admission alone.
Sections dedicated to Cucumber and Rails follow on to close the book. I found the Rails section particularly useful having not previously gotten on to the RSpec 2 bandwagon with Rails 3. There are several chapters that each walk through a particular topic, like view specs, controller specs, and model specs. I didn't want to digest the entire set at once and the structure helped me just dig into the parts I was immediately interested in without following each chapter in order. The large number of short and sweet code examples also helps if you're just scanning through looking for some guidance.
In short, I recommend The RSpec Book. The other reviews here seem to be rather mixed so you might want to check them out to get the bigger picture, but I've found the book to be rather useful with its direct narrative style, logical structure, and vast number of short code examples from which to descry some handy techniques.