Light and Shade by Mary P. Merrifield
Every so often I find myself realizing that I need to override what I am actually seeing in regards to light and shadow shapes in a painting in order to emphasize the form of the objects.
This is where this book is especially helpful, because it is a quick guide to the classical approach to rendering three-dimensional form in the western tradition. The book is organized by the different shapes that you may need to shade, and offers practical advice on how to render a three-dimensional form on a two-dimensional picture plane.
It’s a thin volume, but well worth having in the studio because it is so descriptive.