Documentation:
Official Docs
http://docs.codehaus.org/display/GEOSDOC/Docume...
GeoServer has very extensive documentation, all wiki-able, so feel free to add to it.
 

 

 
Useful Links:

 

 
Training:
The Open Planning Project
http://topp.openplans.org
The Open Planning Project (TOPP) is the main sponser of GeoServer, and offers professional training.