Resources for Nine Men's Morris
There's some information available in German. The game's called "Mühle" (= mill) in German,
and there exists at least one book on this subject.
The book can be found online here: http://www.hubbie.de/
Click on "Mühle", the book can be viewed in html or as an applet.
It's a beginner's book, but contains even some advanced strategies.

It should be possible to understand a lot of the strategies even without knowledge 
of German, but with "Google translate", or dict.leo.org you should be able to make
sense of most of the text.

There's even an organization which organizes (European) championchips, see
http://www.muehlespiel.eu/
It might be possible to get information from there, too.