sharke
The Groove3 course is great for learning the basics of using Reaktor, loading synths, using snapshots etc, but be aware that it doesn't really touch upon building synths in any detail. I guess that would be a whole new course in itself.

A very good introduction to building Reaktor stuff is the Reaktor Master Class taught by Don Philips, aka slamanderanagram.
 
I bought it and his Reaktor Core class.
 
I've gone through about half of the Master Class and I found it very rewarding.
His pace is fast and he talks and builds at the same time.
 
I know most of this stuff fairly well, but I learned a lot seeing how someone else uses Reaktor and Don uses it like, well, like a Master.
 
Read about it here:
 
http://adsrsounds.com/product/courses/native-reaktor-5-masterclass-learn/