3 stars for someone with experience, 2 stars for someone without. There's useful information in here, but it's information you can get for free and with instruction through this open-source program: http://www.lenmus.org/en/phonascus/intro. The most useful section of the book for me was the section about instruments, how they are classified, a little bit about their history and how music for them is notated. As a guitar player and bassist, I know very little in this regard. That was helpful and I don't know of a quick online substitute for that info. It is this section - as well some of the terminology and etymology of musical phrases - I found entertaining.
But be mindful of this book. The copy I have has a typo on the front cover - "Elementary Musictheory" it says instead of "Music Theory," and in one of the charts of the book it says C - Eb - G - B is a Cm7 chord. Not even close. I'm assuming I bought a botched copy somehow.
It's fine as a refresher on that which Lenmus already provides, and can fill in some good gaps in some of the information you already have, but whoever was in charge of printing it ought to have swept the text for typos as egregious as mistaking C - Eb - G - B for a Cm7 chord. As long as you can catch those mistakes, it's a fun little reference guide for when a computer's not handy. Otherwise, use Lenmus: it's free and it has exercises, which this book does not.