For each popular book in some fields there may be an instructors manual, a student study guide, and a test bank. As a grad student, I got paid $750 a long time ago for co-authoring an instructor’s manual with my professor who wrote the textbook. Supplemental materials for popular texts often are written by faculty at smaller schools who get paid a small amount and get some acknowledgement in their annual review. Most of these materials are online these days. I have friends who have co-authored test banks.