Okay, I read this one again, since the first reading was before I knew what things were and why I was doing things and oh, god, what have I gotten myself into? Helps to have development under your belt for these things.
This book was recommended to me as the book to learn Rails development. It has two main sections: a hands-on let's-build-a-rails-app section, and a here-are-the-explanations section. The first is designed to guide you through building an application with Rails, the second to explain what the hell just happened and how all the parts fit together. The book builds a slight e-commerce platform: a store's product display / catalog, administration section, shopping cart, checkout and user authentication. The example is great, I definitely liked that it wasn't a blog or a twitter client.
The book is good and it works. It is not the book for new developers, however. It uses a lot of words and concepts without explaining them, assuming the reader knows what they mean. And this is okay. The book is for a developer who is learning Rails. It does that well.
Time-wise, if you're going to read the book and do the exercises (say, a new employer is giving you a chance to come up to speed with Rails, or the first couple weeks of work to learn), give yourself 2 work weeks to go through it. You can do it faster (you can always do things faster), but doing the exercises and typing things in and playing around with things relevant-to but not part-of the lessons is what makes learning better. So, yeah, play around, poke around, learn something outside the lesson for greater good.