Overview
Drupal documentation relies on volunteer effort. The current documentation team coordinator is Addison Berry.
There are six separate Handbooks, each with its own focus.
The Getting Started book provides some information on the Drupal project, its history, mission and principles. Its main focus is also on setting up Drupal core and some basic best practices. It also has a link to a PDF of the core documentation.
The Getting Involved book provides information on the many ways to get involved with the community. It covers much more than contributing code and attempts to walk users through the steps needed to accomplish things as well as giving context for how the Drupal community operates.
The Beyond the Basics book is a variety community contributed videos, slides, tutorials and how to articles for getting more out of your Drupal site. Even more detailed installation help and other contributed articles on how people solved a need. Other PHP, theme and SQL snippets and tricks on how to leverage the power of your Drupal site can be found here as well. Some contributed modules are documented here as well.
The Theming Guides are about Drupal's very powerful theming capabilities and how to customize the look of a Drupal website.
Developing for Drupal is the oldest book and the original root prior to the division into five handbooks. It is fairly actively updated in various sections as many of our active developers understand the value of documentation. It is hoped that the new Drupal dojo project will also be a good source of new developer documentation. The most up-to-date code documentation and examples can be found on api.drupal.org which is primarily generated from the Drupal source code itself.
The About Drupal book contains information on Drupal version numbers and some information about Drupal.org. It also contains some marketing material and links to professional support services and other tips for interacting with the community. The About Drupal documentation section contains information about Drupal documentation projects, authoring guides for documentation, and copyright and licensing notices for the handbook.

Make the docs searchable
It would be good if there was a way to search only the documentation for Drupal rather than the whole site including forums, etc. Unless I've missed something in the volumes of info on the site (which is one of the reasons I decided on Drupal), there's no easy way to limit searches to a specific area of the site.
you can search handbooks
according to this, http://drupal.org/node/118536