Chapter 1, Developing for Drupal 8, provides an introduction to module development in Drupal 8. In doing so, it introduces the reader to the various subsystems and outlines the requirements for running a Drupal 8 application.
Chapter 2, Creating Your First Module, gets the ball rolling by creating the first Drupal 8 module of the book. Its main focus is to explore the most common things module developers need to know from the get-go.
Chapter 3, Logging and Mailing, is about the tools available for doing something every web-based application does and/or should be doing, that is, sending emails and logging events.
Chapter 4, Theming, presents the theme system from a module developer's perspective in Drupal 8.