If you are serious about developing using Ruby on Rails: buy this book. There is no doubt it will come in useful, even if you consider yourself an advanced RoR developer. I was very comfortable with RoR before I read this book, expecting to only find a few bits and pieces useful, but the level of detail is really great, explaining exactly how each line works, helps both novices and expects alike.
If you want to learn by example, this book might not suit you, as it is more of a reference book than a step by step guide. What it will give you however is a great, deep knowledge of Rails allowing you to 'invent' your own examples along the way. My advice to novices: choose yourself a basic project to begin with (e.g. make a address book system with login), and then read the book with the idea of "how do I do this best?", make notes, then when you feel confident - put all the pieces of your system together.
My summary:
This is the only RoR book I have read that goes beyond the information you can find on the internet and is actually worth buying. You will not be let down. You will refer to it all the time.