I only got into Ruby on Rails in the last year and have been incredibly impressed with it and enthusiastic to grab any book I can to increase my knowledge of it. I was told that Agile Web Development with Rails was "the" Rails book to own, but the only problem was that the fourth edition was not out yet. I looked through a first edition at the local library, but took it with a grain of salt as I knew a lot of that information was outdated, and so, the wait continued and other Rails books I checked out. Finally, this was released and I have now read through the entire book.
It is a little unique over Rails books due to the order of things. Where all the other books tend to start off explaining all the features, terms and and functionality behind the framework and starts working you through a demo app later on, Agile Web Development goes in the opposite direction as it starts you out immediately building the demo app, then the second half of the book goes into the specifics of Rails in more details. As anyone who has read any other edition of this book will already know, you will be building a demo app of a e-commerce store. Although it's not to completion, that's not really the point of it; the app will take you through a handful of different Rails 3 details and by the time you're done reading - or building along - you should have a fairly good grip on how to do different things in Rails. Once in a while, people in the Rails community like DHH and others will chime in with thoughts and tips.
It's a little hard for me to review this book as a complete stranger to Rails, as I have read through other Rails books prior to this. But while I might be more of a fan of the Apress Beginning Rails book, what was great about Agile Web Development was that I still was learning new things like stuff I didn't know you could do in migrations, formatting helpers, etc. So my time with this book was well spent and I am glad to have this on my shelf. Probably the only ones I would not recommend this book to is advanced Rails programmers, especially as they may already own an older edition of the book and I feel they would have caught up to speed on the Rails 3 changes by now. So for the most part, newbies only.
However, I have to point out one thing. This book took a while getting out to market and only appeared a few short months ago, and already it may be outdated...kind of. Rails 3.1 is coming - likely by the time you read this - and there are some new additions that this book won't even cover, such as the new asset pipeline, changes to migrations, SASS and Coffeescript. On the other hand, it's not like the information in this book is suddenly worthless, 99% of it will likely still apply to your projects. It's just something to keep in mind as you're reading this book; I recommend finishing this book then hopping on the web and watching a few Railscasts on Rails 3.1 to fully catch you up.