The Tiny Book of Rules

The Tiny Book of Rulesis tiny and it's about the Rules module for Drupal.

Rules is a module usually used to auto-mate actions on your Drupal site. Rules can react on events occurring on your site - such as a user logging in or a node being created - and perform customized follow-up actions such as redirecting to a certain page or setting field values. Optionally, Rules can evaluate conditions before executing any actions.

Rules is also a framework to use by other modules, allowing them to evaluate user-configured Rules components, such as action and condition sets, or expose data about variables and entities to other parts of Drupal.