MinistryofDrupal.org

http://drupal.org/handbooks

This is the Drupal.org master list of handbooks available on the site in electronic format. Each handbook is a collection of well-written, continually updated, cross-referenced pages. An enormous variety of subjects are covered, including the basics concepts and terminology of Drupal, installation instructions, administration, working with content, building sites, configuring themes, and even coding.

These handbooks are not downloadable as a whole document, but you can of course save the pages for local storage.