From what I understand of the module system in Java 9 I will be required as a daily part of my Java 9 coding to be more explicit in stating the dependencies of modules on one another.
Do you this requirement will generally force, help, or encourage me to be a more modular thinker or will I be somewhat unaffected by the new module system unless I intentionally try to modularize my code? Effectively, will I gain many modular thinking skills or at least familiarity just by using the Java 9 module system?
As to your question on modular thinking, I think it's important to underline that using modules in Java 9 is completely optional. The classpath is still there, and you can ignore modules if you want. So yes, unless you intentionally choose to use the module system, you won't be thinking more modular.
That being said, if you adopt modules, it's not a guarantee that your application is actually modular. That's why we focus on patterns of modularity as well in our book. Modularising a codebase takes deliberate practice and effort. Yes, the module systems provides essential tools (encapsulation, explicit dependencies, services), but it's how you use these tools that matters.
(p.s. did you also see my latest What's New in Java 9 (http://bit.ly/java9-ps) Pluralsight course?)