As they explain at the outset, this book is intended for programmers with some experience with C# and .NET (VB.NET experience should suffice, too) and who have worked with XNA before. You don't need to be an XNA expert or a .NET guru, but if you try to dive into this book without some .NET programming experience and some familiarity with what XNA is, how it works, and what the basic flow of an XNA game is (from the Game-derived class's constructor to Initialize and LoadContent to the main game loop's Update and Draw cycle), you will likely find yourself struggling to understand what's going on. Microsoft's App Hub site contains a lot of great, free educational materials that will help you get started if you are new to XNA and there are other great resources out there if you are new to C# and .NET.
What this book does, and does very well, is introduce you to all of the features of XNA programming on Windows Phone 7, including all of the phone-centric features like accelerometer-based controls, multi-touch input, using notifications, and connecting to web services. You are introduced to much of the material through three, fully-functional sample games, each making use of different functionality and features so that you can see everything come together into an actual game and see how it is integrated. I think that's one of the central values of this book. A sample showing how touch input works and how you get data from it is useful to understand touch input. To make a game, though, you need to see and understand the extra steps of connecting that input to your game, using it to update objects that then interact with the game world (perhaps colliding with an obstacle in the road or sliding out of the way just in time), and then bringing that all together on the screen while playing sounds effects and music. The examples in this book show you how to build the components and how to integrate those with each other to form actual games.
I know George and Chris (if you look closely, you'll notice my name in the dedication section). I think they wrote a really great book. It has helped me at times and I hope it helps you should you choose to purchase it. I definitely encourage you to explore XNA either way. Making your own games is a lot of fun and XNA is a great way to get started doing it.