Debian is a distribution, it is made of many thousands of (free) software :
Each software provides has its own documentation.
Under Debian (Unix), a program's documentation typically consist of (one or more of):
A manpage (type man followed by a command name).
A README file, provided by upstream. (A text file located at /usr/share/doc/pkgname/README)
A README.Debian file, provided by the Debian package maintainer. (A text file located at /usr/share/doc/pkgname/README.Debian)
Programs with a graphical user interface (Gnome, KDE...) usually comes with help page (accessible through Menu > Help).
Some program provide their documentation as html, or info page.
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>.
The most famous for Debian users are :
The Release Notes
The Debian reference
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.
http://www.debian.org/doc - Debian documentation home
http://www.debian.org/doc/manuals/reference/ - Debian Reference Manual
http://www.debian.org/releases/stable/installmanual - Installation manual
http://www.debian.org/support - support home
http://lists.debian.org/users.html - User mailinglists
http://forums.debian.net - Debian.net Forums
Advocacy, developers, inside information
UnofficialRepositories - a list of non official package repositories. They can be very useful.
http://alioth.debian.org - Project hosting for Open Source developers
http://mentors.debian.net - Public package repository
http://planet.debian.org - Planet Debian, aggregating lots of Developer blogs
http://planeta.debian.net - Planeta Debian, a blog aggregator in spanish
http://release.debian.org - Debian Release Management pages
http://snapshot.debian.net - Debian packages archived daily since 2002/06/04
http://times.debian.net - Debian Times
http://www.backports.org - Backports for the stable distribution
http://www.debian.org/News - Official news
Debian Related Wikis
Many free and non-free books are listed on Debian website.
http://www.debian.org/doc/books - Debian Book Library
Extra books could be listed below.
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 firstname.lastname@example.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
This is free software, you can contribute improvements to the documentation
(i.e you don't have to write your own Howto !).