Most documentation and sources around installed by default, I believe.
You can add source repositories to your /etc/apt/sources.list file. Typically you only download the sources you need by doing apt-get source packagename
. Here's a link
to working with source packages in Debian.
What documentation are you looking for? A good help site? Or some specific guide? Debian's website has a lot of their documents in html or pdf format that you can download. A good help site is aboutdebian.com