Having learned the Issue tracking module, you may consider that you have actually learned Redmine, as issue tracking is its main feature that most people work with. Other modules are, in fact, not as critical to know for a usual user, and, as it has been mentioned, the entire Redmine interface is quite easy to understand. However, as this book is not named Redmine: The very basics, I assume that you want to learn more to be able to not only use Redmine but also master it.
The next chapter is intended mostly for project managers and site owners, as it describes the capabilities of Redmine as a project management and a project hosting tool. But it should also be interesting for usual users as it teaches how to navigate through the project and how to find information. Additionally, it shares some best practices.