The default browser home page shows links to a lot of online documentation. Also, KDE and GNOME have help centers for bringing up the documentation for individual programs. For console programs and config files, you can try "man <topic>" or "info <topic>". Info and manpages are also available in the konqueror web browser if you enter "info:<topic>" or "man:topic" in the address bar.
This site has "Linux Tutorials", "Linux Links" and "Linux Wiki" links. A great site for Howtos, and online books is www.tldp.org
( The Linux Documentation Project ). Also, look in your package manager and search for "doc" for a number of packages that install docs. You may have a package that even installs all of the "tldp" site's howtos.
Most local documents are installed under the "/usr/share/doc/" directory.