Debian is a distribution, it is made of many thousands of (free) software :

Program documentation

Each software provides has its own documentation.

Under Debian (Unix), a program's documentation typically consist of (one or more of):

Package Description

In the package description you can find some useful information, like a short description of the package, a link to the original website, a list of files contained, and more. The package description can be accessed at the url http://packages.debian.org/<package-name>. Or, try "aptitude show $PKG" (you may need to "apt-get update && apt-get aptitude && aptitude update" first).

One of the best places to look for help for a single package or application is to read stuff in /usr/share/doc/<package-name> (especially README.Debian). This is documentation written by developers or package maintainers. It is quite concise, but usually mentions most common problems and solutions, and new features and fixed bugs.

Bug tracking system

Many problems' solutions can be found as bug report responses. A search engine for bug reports is found on http://bugs.debian.org. Remember that if you want to search in archived (quite old) bugs you must chose "archived bugs" option. If you want to look at bugs of a specific package use http://bugs.debian.org/<package-name>.

Debian Documentation

The most famous for Debian users are :

Many other Debian Documentation are available at www.debian.org/doc

At the moment, Debian don't have a System Administrtor manual. However, there are some books about Debian, see: www.debian.org/doc/books.

Most documentation for Debian-developer (and package maintainer) are available from www.debian.org/devel/

Official Websites of the Debian Project

On Debian.org the official Debian site, you can find a section dedicated to documentation.

Forum

Advocacy, developers, inside information

Wiki

Debian Related Wikis

Books

Many free and non-free books are listed on Debian website.

Extra books could be listed below.

Instant Messaging

IRC channel

An IRC channel named #debian can be found on irc.debian.org and on irc.freenode.net. To connect, you need an IRC client (see IrcClients). More information can be found on GettingHelpOnIrc and IRC.

The channel has its own FAQ.

Jabber chat rooms

There is a chat room dedicated to Debian support on jabber.org server. Join the room at debian@conference.jabber.org with the jabber client of your choice.

More information on Jabber and Debian support chat rooms for your own language.

Contribute to documentation

Most of the Debian Documentation is available at http://www.debian.org/doc/ddp