All too common among self-taught artists (as well as some trained ones) is a lack of edge variation in their work. Although the opposite also occurs, this error is most often made by representing all edges as being equally sharp. But to make an accurate representational statement you must use the full range of edges and this begins with finding the extremes: the sharpest sharp and softest soft.
Context is everything, or so the saying goes, and it's so familiar that it's easy to quickly skip past it. But stop and think about it for a minute. In any given scene all values are unconsciously perceived relative to every other value.
Mentioned before is the fact that Sight-Size is as useful for drawing all visual subjects as it is for cast drawing and portraiture. Why? Because it is the only arrangement which provides the eye of the painter with a one-to-one comparison between the subject and artwork.
During the late nineteenth and early twentieth centuries numerous artists and instructors wrote instructional manuals attempting to show the general populace how to draw. Many of those authors incorporated aspects of Sight-Size into their teaching. Edwin George Lutz, author of Practical Drawing, was one such popular arts teacher.
Although Sight-Size is most often discussed as an approach to portrait painting, Sight-Size is not just for portraiture. In fact, the first written use of the term relates to figure drawing.
I would imagine that all teachers have a list of recommended books which they consider to be essential reading for their students. Mr. Gammell was no exception. This article passes that list onto you. My sources are a half-dozen of Mr. Gammell's former students.
Mentioned many times on this site is that you will draw better when you have an accurate eye; that learning to draw is actually learning to see. To help you with the process, in early 2016 I wrote an ebook, titled An Accurate Eye. Recently brought to my attention is an online eye accuracy trainer: The Eyeballing Game, that nicely supplements the book's exercises.
Depending upon how deep you want to go, the principles of light and shade can be quite complex. Nonetheless, everything hinges on the fact that light travels in a straight line. Upon hitting a surface light also reflects back in a straight line. How much of that light reflects back to your eye is dependent upon whether the surface is in the world of light or in the world of shadow.