allmusic are pretty much the top source for getting a foot-in into any style of music from any country or time period, with their reviews being used as the basis from all kinds of things from the iTunes store to Spotify, and this book is almost certainly invaluable to anyone looking for an introduction or anyone who just wants to fill in the gaps in their knowledge. However, it seems like every single word of this book is taken from their frankly fantastic website. You can't argue with the content found here, but unless consulting websites for info is not already an everyday part of your life you might as well just browse the site for free.
This pretty much applies to all books in the allmusic guide range. For people who could just as easily use the website, maybe 'The Rough Guide to Hip-Hop' by Peter Shapiro would make a more worthwhile companion. I haven't read it, but it looks pretty much the same as this book and at least it's an alternative take on the subject.