https://www.drupal.org/docs/8/extending-drupal-8
https://www.drupal.org/docs/8/api
https://www.drupal.org/docs/8/creating-custom-modules
https://api.drupal.org/api/drupal/core%21core.api.php/group/hooks/8.6.x
cmslaboの勉強会で行ったモジュール開発の資料
http://cmslabo.org/sites/default/files/doc/workshop/2015/drupal_workshop...
http://cmslabo.org/sites/default/files/doc/workshop/2017/0419/drupal_wor...
https://www.drupal.org/project/module_builder
https://www.drupal.org/node/205585
その他Drupal8のモジュール開発、いっぱいあるようです。