Also, if you want to spend a few dollars, there are a bunch of great RedHat/Fedora books.
SAMS RedHat Linux & Fedora Unleashed is a good one, I have an old edition of it lying around at work.
Also, for fedora:
http://fedoraforum.org
http://fedoraunity.org (check links on left side of page)
Different distributions have different tools and methods, so learning Fedora inside and out might not help you a lot with something like Debian when it comes to system configuration.
I recommend you get good with one distro (Fedora is fine), and when you start to get antsy and want to learn more, install Debian or something else in VirtualBox or get VMWare (Player or Workstation) to play around with other distributions while still having something you're familiar with for daily use.