I only received my copy of the book about 2 days ago, but so far it has been one of the most impressive resources I've found on Android development (not just books, but across the web too).
I come from a heavy background in web development, specifically PHP (call it a language or a mess, but it's my web programming language of choice). I've wanted to move into Android development for a while, but never really had an opportunity until recently (3 months ago) to produce something. Naturally, moving to Android development was rough at first for countless reasons, so I've bought books, read tutorials, blogs, watched countless Google I/O videos, and spent hours upon hours browsing the API. For the most part, I've found "here's how to do (insert random task here)." Rarely have tutorials come with the reason something is a best practice or why their way is better than another. Since this is the case for a lot of languages, I mostly accepted it and did my best to make sense of the content out there to build my first app.
A great reference and more:
In the two days that I've had the book, I think I have saved at least 5-6 hours that would have been spent looking for solutions to problems that I felt comfortable implementing in my software. Not only does Reto explain how something is done, but more often then not I feel like I understand why something is done a particular way.. Which, to most developers is crucial to get an understanding of a platform. I would be comfortable saying, there's enough information out there for ANYONE to produce an Android application, but for non-Java programmers there are few resources that really solidify a better understanding of the platform or don't recommend a life-long dedication to Java before picking up the material. I'd feel comfortable recommending the book to anyone with a solid understanding of software development who was looking to get into Android programming.
To be more specific than saying "a solid understanding of software development," I'd say it definitely helps to know some Java syntax, object-oriented programming and MVC practices going in.. Of course, the more the better. Without footing in those a lot of the information (not just in this book, but Android development in general) will be tough to grasp completely. After that, I'd say it's for just about anyone serious about Android Development who has more interest than just getting from A to B. Though it would certainly help anyone just looking to get from A to B.