A guide to all the stuff you want to know about Drupal 8 module / theme / profile development, but didn't know you needed to know.
This is not meant to be a full fledged tutorial to train you in building modules. Rather, it is a quick reference "cookbook" targeted towards those of us who aren't completely immersed in Drupal development and need to look stuff on a regular basis. It provides the "recipes" for proper module development, which you can then season to taste.
The cookbook is also not meant to be exhaustive - it's just meant to cover a lot of the most commonly used pieces and parts of Drupal module / theme / profile development. For all the nuts and bolts of Drupal 8, see the Drupal 8 Documentation site and the Drupal API site. (Keep scrolling down on the documentation site - there's a lot more useful stuff hidden below the large "Contributed Modules" section.
There is also a Drupal.org "Converting Drupal 7 Modules to Drupal 8" guide that could be useful to module builders as well.