The relatively new GTQE was created by the BLM and IMBA. A link to the document is here: GQTE
Dirt Rag has an overview here: Access: How to build friggin’ awesome mountain bike trails (that are actually legal)
It is a nice document, but don’t get bound to tightly by the pages.The Pennsylvania Trail Design & Development Principles is very nice, and more broad in its scope.
More Trail Building Resources here.