Drawing was on my list of things I want to know how to do for a long time. A year ago I've seen a nice recommendation of this book saying that the science in it is not worth reading, but the exercises are great.
I have to admit that I skipped the exercises in last chapters - basically I don't have crayons or pastels and I don't want to buy them right now. Besides that, I have learned a lot.
Exercises were really hard for me, even sitting and drawing for an hour was really hard. Fortunately in the end I have set a goal to myself to finish it in June. Without that specific goal, it would probably never happen.
After finishing my self-portrait in chapter 10 and comparing it to the one before the book I was amazed. Don't get me wrong, the people on both on them don't resemble me, but the third one at least looks like a person.
Most of what I learned about drawing would be something I would consider "cheating". I though that artists just happen to sit and make the drawings from blank paper. What's happening is (to my understanding) more like taking a photo in your mind and drawing it on paper. It was great to see the early works of famous artists with the same errors I've made and learn easy to apply formulas how to prevent them.
If you want to learn how to draw I can recommend it as a starting point.