As one of the dummies this book was ostensibly written for, I can say I found it quite useful. Last week I didn't even know what a treble clef was for, or why a piano had black keys. After reading this book, I get to look at my past over the shoulder with a little sense of superiority.
I found somewhat distressing to learn that music theory seems to be choke-full with historical artifacts. Of course, that's not the book's fault: there's nothing the authors can do about it. What they do is to include some instructive history asides that help you understand why things are the way they are, rather than just telling you "this is what you need to know, period."
There is, however, something to be forewarned about. The book's title might lead you to think this is some kind of primer, which is the reason I picked it up in the first place. Instead, it is written as a reference guide, so reading it from beginning to end may be a sometimes tedious experience. Every specific topic has its own heading, where it is thoroughly elaborated upon before moving on to the next. You don't get to read anything at all about harmony or melody until everything has been said about beat, tempo, bars, and time signatures; and you don't find a single word about chords until after every possible perfect, major, minor, augmented, and diminished interval has been painstakingly described and shown to you. And yes: some of the discussion seems to be a tad above the "for dummies" level promised by the cover. I doubt someone who just wants to get a better sense of what their guitar instructor means by "this is an E7 chord" will be building a minor harmonic scale very soon.
In short: yes, the book is good and useful. However, like music itself, might have benefited from a better-planned progression.