Not terrible. It had good information about the stages of development, but I feel like it easily could've been explained just as well in a pamphlet. It is designed more for choir directors than private singing teachers. Quick, bland read. Somewhat helpful.
Still one of the foundational texts for proper understanding of the adolescent singing voice. This is a good starting point before diving into more recent research.
It's a bit dated, but still has a lot of useful information. The appendix in the back that describes each stage of adolescent voice change is particularly helpful.