I finished reading most of Agile Web Development with Rails, fourth edition. This is the third edition that I've bought and read. I still love this book.
This edition is much shorter than the previous editions. I tend to read books cover-to-cover, so I appreciate when they aren't full of reference information that I can find online. I think this book struck the right balance of breadth, depth, and length. My buddy Shailen Tuli thought it was too short. Your mileage may vary.
I think the only thing bad about this book is that it focuses on Rails 3.0. Because Rails 3.1 has something called the "asset pipeline", it's not so easy to make the jump. I decided to stick with Rails 3.0 for the time being because I really like having a book to refer to.