I got a fair bit out of this book and successfully pulled off a few paid gigs thanks to it and a free online course (that was a little more practical to be fair). I was looking to draw caricatures live, which is 5 minutes per drawing. This book isn't targeted at that audience so is reviewed with that in mind.
It's not the greatest book and one person's style, but does give you enough to start you off in drawing good caricatures and developing your own style. I used this in conjunction with some good online courses and do recommend it as a resource, even if live caricatures aren't your thing.