There are few in-print books on diagramming. Ed Tufte is the authority on the visual display of quantitative information, but there are other types of diagrams. A fine book is The Back of the Napkin by Dan Roam (
http://www.thebackofthenapkin.com/). Dan's free hand approach is great for brainstorming but lacks a formal grammar and rhetoric. I call this sketch to sell [an idea]. Diagrams that can be used to visually analyze ideas and processes are more formal. I'm working on the latter, but have no book yet. Feel free to post here or email me questions, and I'll try to help; at least until I get flooded. <g> If I do get flooded, that might help the book happen. <g>