I have been toying with Rails for several years, and finally decided to get serious about about 4, maybe 6 months ago. I browsed this book, and dug the approach it took, so I bought both the physical book and the ebook.
The book walks you through the development of an app, step by step, and most importantly, includes testing. This is a subject that most books gloss over with, "normally you would write tests for this", or "in the real world you would probably want to write test". Some of the better books may devote a chapter, but don't really give you practical experience in writing tests as you go.
The approach the book takes is nice too in that it does just enough hand-holding. You're given step-by-step instructions when you need them, and referred back to previous reading if you should have learned something already. It starts with pretty basic topics, and works its way through to more advanced topics.
I would recommend going through the book, doing a couple of apps on your own, and then going through the book again to "get it". Well worth the read.
While the book does have a lot of errors, it also has a great support community behind it. Every problem I ran in to I was able to quickly find a solution to on the book's forum.
My problem is with the publisher. One of the reasons I bought the ebook was that, well, ebooks can be updated! No errors! Maybe with other publishers, but apparently not with Manning. Very weird. The cost can be that great. Tech readers expect it, but Manning doesn't get it.