This is a good book, explaining chord music theory. The main reason I like this book is because it explains music theory in plain English, without the use of jargon, if possible. If any jargon is used, its meaning is explained in plain language. This makes the text easy to follow by beginning musicians.
In a nutshell, this book explains music (mostly chord) theory an average, budding amateur musician should know to get the most enjoyment from music. The book is not a comprehensive explanation of all music theory. For example, it doesn't explain how to read musical notation. It confines itself to how chords are constructed and their relationship to different parts of music (melody, etc).
It is slightly orientated to keyboard users. However, the books content is still 99% applicable to other instruments (I play the ukulele).
As I've already written, the main strength of this book is the plain language, easy to understand explanations. That's the main weakness of other music books. Their explanations usually involve jargon which lose the new musician very quickly. For an example of what I'm talking about, look up "chord progression" in Wikipedia.
A major benefit of reading this book is I'm now able to understand other music theory books I was struggling with before.