I just finished reading this book: The Back of the Napkin - Solving Problems and Selling Ideas with Pictures by Dan Roam. It currently has 4.5/5 stars from 48 Amazon reviewers. It was OK. I was hoping for more. The focus of the book seems to be to instruct the reader on how to employ simple diagrams in a variety of applications. It provides an orderly framework for choosing from various representational diagram styles allowing the reader to construct their own persuasive illustrations. My low opinion might be partly biased by my personal experience. I have a lot of experience producing illustrations and other artwork. Heck, years ago I sold my art (mostly pencil and charcoal drawings) and later in life I was lucky enough to dabble at technical MARCOM (marketing communications) when I was an engineer at TRW.
So, if you're illustration-challenged and are looking for some way to improve, then you'll get a lot out of this book. Otherwise, I'd suggest skipping it.