Up front: I'm a bit biased here as I was the Technical Development Editor and contributed a bit to this book. However, it is an excellent book.
When I started reading, I wondered how one could write a whole book about the Java Module System. Well you can. This book reminded me a little of the O'Reilly Regular Expression book. If you want to understand the topic just enough to get started, you can read just a little. If you want to understand it in depth, you can read the whole book.
I really liked the examples and migration strategies in this book. I didn't know much about the module system starting out. By the time I finished the book, I knew a good amount. Then I didn't use it and forgot.
This year, I took both OCP Java certification exams. As both covered modules, I used this book to study the topic. It came back right away and I am recommending chapters in this book to those studying for the certification. (Until study guides come out.) Conveniently, Manning allows buying parts of a book.
If you want to learn about modules rather than just pass the cert, you should buy the whole book though! You'll learn a lot; even in the appendixes.