Those are beautiful drawings. I have no idea/talent for how you get to that level but I can draw with some degree of skill.

I'd say that a book that best explains how it works is "Drawing on the Right Side of the Brain". I don't know if it's still in print but it was a college textbook of mine and it crystallized my interest in the medium. It's really about how you look at things. Really look at them and break them down into geometry and symmetry and what you are actually seeing. I'd almost guarantee that artist never got a twinge of a hard-on while drawing that beautiful woman, and he probably knows her body now better than her S.O.