Summary: Great book after you've learned the basics of RoR.
I'm writing this review because I disagree strongly with some of the previously submitted reviews that rated this book poorly.
This IS the book I'd HIGHLY recommend to anyone with a programming background who has gone through intro-level RoR books and online tutorials and wants to delve deeper into understanding the framework, in terms of gotchas, tips, recommended coding practices, etc.
When I go to a book store and pick up a book, I skim through it, look at the content, and see if the author(s) covered important or complex topics with an appropriate level of detail. This book nailed that part. Even in skimming the book for 5 minutes, I found explanations for several issues I had run into while learning RoR. E.g. with a has-one / belong-to relationship between 2 models, when does the relationship get saved if you associate the parent in the child, or associate the child to the parent? Things like that, which are relevant to programmers build real applications, are invaluable to know.
One thing that makes the book excellent is how well it explains options and their tradeoffs for implementation of functionality. E.g. there is an excellent writeup on the options for managing session data.