The best way to get to know a technology closer is to study it's code. Drupal 8 is a huge step forward technologically from Drupal 7, and it has literally tons of code that I feel I need to review and learn from. And I welcome comments and crtiticism from the readers.
The module that I chose to start with is a simple one: the Ban module. It's used to ban IPs via interface and first appeared in Drupal 7, undergoing a rewrite for Drupal 8. Why I chose this module? Because it's simple. We'll take the Drupal 8 module as an example and lay out it's anatomy and of how a Drupal 8 module is built, and try to explain some new concepts with it. And, you can always get Drupal 8 from drupal.org and see the code yourself, and play with it.