Behavior Driven Development is an important new methodology, and the authors of The RSpec Book provide a solid introduction to the theory and practice of BDD. However, I feel this book would have been just as good as a series of blog posts. This is not meant to denigrate the book, into which I know much effort was invested. However, the examples in the book fail to tackle the real-world challenges that you will face when implementing BDD.
Einstein said "Everything should be kept as simple as possible, but not simpler." A common fault of software courses and books is an avoidance of real-world complexity. For example, in The RSpec Book, the last 3 or 4 chapters are on BDD with Ruby on Rails. The sample application that is developed is ridiculously simple. Also, no cucumber specs are developed for it, so we are basically writing code for its own sake, rather than executing on the BDD mantra of "writing software that matters."
Both BDD and Ruby on Rails are meant to offer solutions for large, complex software projects and the ins and outs of their proper usage can only be learned by application to software that goes well beyond toy functionality. In a large Rails projects, with dozens of models with complex associations interacting with multiple gems, managing RSpec examples and Cucumber scenarios is a project in and of itself. The introductory example application "CodeBreaker" is better because it shows the full BDD development cycle with both cucumber and RSPec. Perhaps the authors should have built on that same example in the Ruby on Rails chapters.
In the end, if you want to learn BDD, you definitely should buy this book. The authors would do well, however, to bring in more of their real world experience in future editions.
One final note for those interested in advice on real-word BDD best practices, google "You're cuking it wrong" by Jonas Nicklas.