I was randomly downloading references for art-studies when came across this book. And well, I was left between "okay..." and "Good Lord! This is dangerous!"
Okay. It's a good book. But. There will always be a but. I realized that in some of the illustrations, the shadows has been done incorrectly. And somewhere the volumes of the things, distorted and so on. it was appalling. I mean, please while using this as a reference material for your practice make sure you are not completely "mugging up" what's been done here. Figure out the light source and likewise start shading the casted shadows.
There are other things which I'd positively take from the book are the line weights since traditionally, it's important. Digitally it can easily be manipulated but it's the traditional skills & knowledge to be mastered first.
Edit. Amazingly easy? Naw. You oughta not teach incorrect information in name of making "art" look easy. I'd say, one can skim through this book. Take whatever you find fit and move on to a better reference book.