Early on, the author points to an interesting statistic that, in 2010, traffic to mobile websites grew 600 percent. This point frames the discussion of what the book's focus is on, mainly that mobile is important, mobile is different than previous mental models, and it is growing at an alarming rate.
The author goes on to enlighten us to the fact that this isn't really a mobile or small screen problem that we are dealing with. This is a problem of connected devices with a wide ranges of screen sizes, device capabilities, and network speeds.
One last big point at the outset, multiple sites based on screen size do not scale.
From this beginning the author introduces the device-agnostic concept which simply put means you can't know what device will be using your site so don't build your site for one device.
At this point a two of high level concepts are introduced: Future Friendly and Progressive Enhancement. These form the base principles discussed through out the rest of the book when discussing Responsive Web Design.
This book covers a lot of ground from planning and workflow through responsive media and server side detection. Implementing Responsive Design even delves into the often ignored topic of responsive content.
The intended audience is beginner to intermediate, one should be well versed in the basics of CSS and HTML prior to beginning this book. Advanced topics are hinted at but not dealt with in any depth so no need to worry. You can safely ignore those topics and come back to them as you build your skill set.
While this book purports to be simply about Responsive Web Design, I would say that it is much more than that. It is a book that you will read and come away with a full breadth of knowledge to use in developing great websites and not a book, like many are, leaving you seeking other books to fill in gaps.
If you only have space for one resource on your bookshelf, then this is the book to fill it with.