By far the best Rails book money can buy, and I've browsed most of them if not all.
First, if you still had doubts, YES you need a book to really get a good, rapid grip on rails. It's moving at lightning speed, in fact a new release showed up a few days after this 3rd edition went to print. A recent rails book is worth a million outdated screencasts and hacked together tutorials.
The review process is worth mentionning. Each edition of this book go through a long beta process where it is reviewed by 'beta readers' - this edition went through 10 iterations for example. This means you'll find far less typos than in other books, and a lot more 'best practices'.
I've taught a lot and I find this book to be of far higher standards than a lot of 'official' material out there developed for closed-source products.
One thing though, this book definitely assumes you've got some programming experience, preferably OO background, so I wouldn't consider it to be a 'beginner book' (to be fair it's not advertised as such). Rails itself is similar to this book - you need to have developed a lot of prior applications to truly appreciate its elegance.
I also recommend you have a look at their website, Pragmatic Programmer, where you can get an electronic copy of the book in PDF and kindle format, well worth it.