What distro do you use? You should be able to get file listings from your packages. It used to be a lot easier to see what was going on when X was in its own hierarchy. It's a lot of files, though, and there are a lot of man pages to go with them, which will help identify what many of them do. As far as online resources, I haven't been to the xorg website in awhile but it seems like they had online versions of the manuals. Your distro may separate the documentation and you'd need to install doc packages but there are a lot of text and/or html files documenting various aspects.
-- Here's a link to the docs, though the man pages should do for individual apps and files.
http://xorg.freedesktop.org/releases/X11R7.2/src/doc/
Searching the web also turns up stuff like this:
http://lists.x.org/archives/xorg-mod...er/000801.html
So there are many resources you can look for and at.