As far as “textbooks” go, the Drawing Textbook by Bruce McIntyre is near the top of the list. It begins with an explanation of the need for a focus on principles in art teaching and then takes the student through dozens of practical exercises to learn and practice the principles. It the perfect drawing primer for all ages!
7 Principles of Drawing:
3- Surface Lines
Where to get it: Audio-Visual Drawing
Use with: Drawing on the Right Side of the Brain