Here is a link that contains several free Andrew Loomis books that have been converted into pdfs for easy downloading. Andrew Loomis is an illustrator and draftsman from the 50s and 60s. The first book listed, Creative Illustrations, contains in depth studies of Line, Tone, Color, and Creating Ideas. I skimmed through the book and thought the material to be interesting and helpful. It offers different processes and techniques on composition as well. I just wanted to share it with all of you. Hope it helps you in some way!