It's always fascinating to look, as much as one can, inside another's creative process, and this book, to some extent, allows us to do so.
But what is more helpful in understanding art, drawing in particular, is the first section, "Text," which does discuss some technicalities, such as negative space and the disappearing edge, but, more importantly, manages to convey and thoughtfully discuss the emotion and intent of drawing. Maslen and Southern make a wonderful case for the importance and meaning of drawing, and the quality of line and marks. They manage to be unaffected and uninfected by -isms.
This is not an instruction manual, but it is a useful adjunct for those learning to draw who want to consider the meaning inherent in line. It is also useful for those who have drawn for some time, as it may make you re-examine your intent and experiences.