This book provides information for Drupal developers on the Drupal API, and is meant to act as a companion to the detailed class and function reference information that you can find on https://api.drupal.org (the official Drupal API reference site). The information here is of a tutorial and descriptive nature, and is built from AsciiDoc source files that you can find in the "Core Docs" project on Drupal.org. (In contrast, the function/class reference information on https://api.drupal.org is built from specially-formatted comments embedded in the Drupal Core files themselves.)