Another primary source of information are the info documents, viewed either with the info command, or via from info-mode within emacs. All GNU programs and much of the rest of our system is well documented with info, easily navigable but as much detail as you might want.
info uses similar commands to emacs. Typing `h' from within info invokes comprehensive documentation about info, including a tutorial on using it.
Typing `q' gets you out of info.