A guide to all the stuff you want to know about Drupal module / theme / profile development, but didn't know you needed to know. Everything provided here applies to modern versions of Drupal (i.e. Drupal 8 and later). Please do note that the cookbook is not regularly reviewed and updated, so some examples may no longer work if the Drupal core developers have changed an applicable API since Drupal was released.
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.
* - This cookbook was written long before the Drupal core developers adopted the term 'recipes' for their own purposes. Within this cookbook, 'recipes' refers to ways to formulaicly build modules, not the more modern components supported by recent versions of Drupal.
The cookbook is 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, see the Drupal API Documentation site.