Since the form alphabet is the basis of art we will spend a little more time on it. Photographs for the most part destroy form because they are non interpretative. The photograph does not create a hierarchy of subordinate forms overlayed on a gestural structure. You must learn to translate what you see using a form language which for the most part you may construct yourself, into an image such that you must become aware when you are drawing in the classical sense and when you are copying like a camera. A professional portrait photographer will use various levels of bounce lighting from a varity of directions in order to illuminate the form of the sitter to prevent the natural forms from flatening out when the photograph is taken.
Artists create form and shape and thus have much more controle over the lighting environment.
They use light to present form and or simulate form under natural lighting conditions. This is the key element that allows you to become aware of when you are drawing in the classical sense and when you are copying in the camera sense. It is currently available in some book stores. Don't focus on the anatomy section - you don't need it, you want to understand how form is created not become a doctor. There has been too much made of anatomical knowledge to the detrement of form.