Unlike what the cover text says, this book is neither very rigorous, nor readable. In the end, you get the idea, but I wouldn't say it does a good job explaining all the choices the authors of the language made (except for the recurring "it was too hard, so we hacked together something that appears to work but really doesn't"). Overall, it feels much more like a collection of blog posts, rather than an explanation of a programming language.
And as for Dart, the language, this book brought me from an initial excitement to disappointment without having to spend too much time with it.