Matt (& others),
The course is designed primarily to explain the real basics to a new pilot and then point them in the direction of more detailed info on the Web, so the e-mails don't cover radio navigation aids in themselves.
They were purely written to help newbies learn a basic level of skills so they can become part of our community quickly

(So the experienced pilots among you who've asked for them may find the content quite basic... )
However Matt, although Rod Machado may not be everyone's cup of tea, don't let him put you off using the FS Ground School and Learning Centre as the info you need is contained in there and it is pretty well explained.
If you really can't get what you want out of those sources a really good overall FS navigation guide is at:
http://www.navfltsm.addr.com/index.htmand this goes into a lot of detail about all the radio navigation aids.
There's also a link in the e-mail course to the FAA's free Pilot Manuals which cover most aviation subjects at:
http://www.faa.gov/library/manuals/aviation/Hope that helps.