Taking textbooks as a standard is presuming the primary purpose of an RPG text is instructional. No way. The primary goal of an RPG text is inspirational. Textbooks exist in a context where reading them is mostly mandatory. If an RPG text doesn’t inspire in some way it doesn’t get played or talked about. No one plays an RPG because the text is rigorously instructional, or because the text is a good in-play reference book.