Andrew Loomis's "Creative Illustration" is the most expensive book I've ever bought (although I think my 3-volume collection of The Zohar is close. I was feeling especially ambitious when I bought THAT!)
Out of all of the Loomis books I own, this one scared me the most. It felt more dense and serious than some of his other books and is the most extensive of the bunch. When I finally decided to crack it open (can't let that money go to waste!), I was glad I did. There's a lot of helpful information in this book. Download the pdf here.
Published in 1947, Loomis divides the book into five areas: Line, Tone, Color, Telling the Story and Creating Ideas.
Here's what Loomis says about tone:
Tone is the degree of value between white and black, the lightness or darkness of a value in relationship to other values. Every object has "local value" which can be brightened or darkened by light or the lack of it. The artist is interested only in the effect of light or darkness on the local value, NOT the local value itself.
The 4 essential properties of tone--
1. Intensity of Light in Relation to Shadow. The relationship between light and shadow completely depends upon the intensity of light.
2. Relationship of value to all adjacent tones. Values is like resizing, a proportional relationship must be maintained.
3. Identification of the quality of light. The kind and relationship of values determines the kind and quality of light.
4. Incorporation of the influence of reflected light. Everything upon which light shines also reflects light. Nearly all shadows contain some reflected light. If you neglect to show the reflected light in the shadow, you'll lose your form's solidity and it'll look "dead." Reflected light helps make things appear round, 3-D.
You only need 4 values with which to work: white, light gray, dark gray and black.
Loomis's analysis is deep. I'll post more of it in the future.