Sorry, I thought I answered this. I probably forgot to click Submit.
Core Java Volume II has one chapter on modules. It contains what I think every developer should know about modules, and some more advanced material that some developers should know.
Every developer should know that modules encapsulate packages, and that the Java Platform Module System ensures that a module states on which other modules it depends. Every developer should also have a basic idea why Java 9 and above give nasty messages even when you don't use modules--reflective access, access to Java internals.
Some developers should know how to modularize an application and how to manage the migration to modules, particularly for libraries that someone else might want to use in a modular application.
A few select developers will also want to know the finer points of JPMS: transitive and static requirements, qualified exporting and opening, service loading.
All this is covered in the book. The book doesn't go into even more esoteric aspects such as module layers that are completely untested in the wild.
And finally, every developer should know that the Java Platform Module System is of great benefit to the Java platform but it is much less clear whether there is any benefit for your application. That too is covered.