The book's preface indicated that the author is well versed in Ruby, and that he got into Rails when it was brand new. It sounded like he was in a position to teach the subject matter well, and I really liked his angle: Rails was written in Ruby, and Rails extends Ruby... you really cannot fully utilize Rails without learning Ruby, and this book teaches you all about Ruby, with an eye on Rails. Perfect!
There are four parts to the book. The first part is an introduction which explained how Ruby works, how Rails works, and why it's important for Rails developers to have a good handle on the Ruby programming language. This was more detail than I had gotten in my online reading, and not so much detail that my head blew up immediately. A real plus. Also in the first section, we walked through the beginning of the development process for a simple "music store" web application. This defined a context for the pieces of the Ruby puzzle that were about to be dumped on the table.
The second part of the book really delved into the guts of Ruby programming. It defined objects, variables, classes, instances, constants, modules, methods, inheritance, scope, and control flow techniques. It had been 8 years since I'd done work in Java, and that was my only experience with object oriented programming. Ruby is kind of the same, but not really.
The third part of the book is all about the classes and modules that are built in to Ruby. This is very important stuff, necessary in order to get a feel for what you can do (manipulations and comparisons) with different data types. And there is a lot you can do. This part also describes how arrays, hashes, and regular expressions work in Ruby.
The fourth and final part of this book re-focuses on the music store application from earlier in the book. Once you've gotten a tour of Ruby, you see Rails development in a different light. Controller and model files look different when you have a feel for what you're actually looking at. With your new perspective, the author walks you through several improvements, bringing the online music store closer to reality.
As a kind of a bonus, the final chapter of the book gives some techniques for learning more about Rails (and really, Ruby, too.) This is very good stuff! How do you search through the Rails code to find the section that pertains to your question? How can you more effectively search online documentation? Answers to these kinds of questions can really help a person grow, because once you become an intelligent troubleshooter, the only thing holding you back is your own level of motivation.