I figured it was best to get this out into the wild while the OGL1.0a was still applicable.
This is a book originally intended to be one of a pair, with everything you need to DM a 5th Edition D&D game in the World of Greyhawk, except the maps.
Contents include Campaigning in the World of Greyhawk, History of the Flanaess, Calendar and Seasons, Weather, Cosmology, Lands and Wildernesses of the Flanaess, Magic Items, and Monsters.
You can download the book here. Enjoy!