This starts at an extremely simple and accessible level. If you've never handled a bare LED before, then it's pitched at exactly the right place to begin. The basics of setting up the Arduino IDE and a "Hello World"-level pushbutton to LED program are given.
The trouble is that this is about as far as the book goes. It assumes you know nothing to start with (a good thing), but doesn't leave you much further along at the end of it. If it were twice the length, then it might achieve more.
There's nothing in here that's reference material. Once you're through this book once (an evening, maybe two) you're finished with it.
There's little inspiration in here. It's not a patch on, "Making Things Talk". It tries hard enough, but there just isn't space. The integration between Processing on a desktop to analyse an RSS feed and then communicate by serial over USB to the Arduino and some LEDs is a good idea, but the clarification between Sketch and Processing could have been made more obvious (just some different typography would have helped).
This is a good book if you're running one-day workshops for kids with no hardware knowledge at all. It does handle starting from scratch very well, it just doesn't go far enough to really spark interest.
If you already knew what an Arduino was before looking at this book though, then you don't need it. Start with the online refs, and keep looking for a really good tutorial to getting started with the Arduino.