I am a professional artist. I began art teaching 6 years ago. My students have progressively gotten older so the challenges of teaching drawing increase. When I first began teachin, I found the Kaupelis book which has been a bible for me. It was also obvious to me that my teachers used the Kaupelis book back in the early 80's.
But back to the "Keys" book, there was one bad review of this book and I want to address that one first. It went on and on about how bad the drawing examples were in the book. If you are anything like me, by the time I get the book I forget what the reviewers wrote specifically. My first reaction to some the drawings was that they were pretty bad, but this book is excellent despite this.
What the first high school teacher writes is right on the money. This book is thorough and I find myself referring to it more than all of my 25 drawing books (smile) I have in this house... and am most enthused with my lessons that are inspired my the "keys", which improves my teaching even more.
He includes words on thought process while drawing. He breaks down drawing to these small, bite size ideas that are very easy to understand and translatable. Of course, as you teach, you learn. This is a book for reading, then application. If you want a book to look at filled with beautiful drawings, I suggest, John Biggers, Charles White and Pontormo.