Debian is a distribution, it is made of many thousands of (free) software :
Each software provides has its own documentation (more below).
Debian has some own documentation too (more below).
At the bottom of the page, there are some pointers if you want to contribute improvement to documentation (more below).
Programs documentations
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.
Debian Documentation
The most famous for Debian users are :
The Installation Guide
. http://www.debian.org/doc/user-manuals#install
. (or development version, available at http://d-i.alioth.debian.org/manual/The Release Notes
. http://www.debian.org/doc/user-manuals#relnotesThe Debian reference
. http://www.debian.org/doc/user-manuals#quick-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/
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 !).