Useful for a overview and medium detail explanation about configurations of Linux components as well as packages. Although package names vary from distro to distro, nearly all of the information transfer.
Useful for a more technical lower level detail of Linux components.
I collection of projects and operating systems with links to their wikis. May not be limited to Open Source, but does include them.
Has a cache of hundreds of manpages, as well as some basics
The Linux Documentation Project
This is rather old, (most recent article dates from 2012), but may have some useful things