Really liked this book, and agree that you must draw what you see, not what you know - e.g for an eye, areas of light and dark and their relative positions, not an ellipse with a circle in it. That said, it is worth understanding some basic anatomical rules - e.g. that a persons eyes are halfway up their head, not near the top.

This is why drawing people is such a good test. If you draw a house or a tree, you can get proportions wrong and it can still look credible. But the same types of error stand out glaringly in a picture of a person.