Is there a reason why "users" was in quotes?
In any case, I am a single-person shop these days so haven't had the issue and I rarely use job jackets and don't know if I ever used one on a book job or not (but I don't think so). But it sounds like perhaps the job jacket used by one person isn't the same as another person, or someone that got that message relinked a job jacket and that messes up the book file (perhaps it was yet different).
When I do create books using a book file, I do create it last, though. Which may explain that even though I am a single user I haven't had the issue.
Is the job jacket on a server and available to all users? Is the path to that job jacket the same for all users? And was the book file created that pointed to that single job jacket?