I wonder if, in the case of Anwyn, we can take a lesson from Shadow World. Sort of get the idea of the broad brushstrokes of the world and release regionalized sourcebooks all with slightly different flavors to them?
The Core book I am working on is pretty much giong to be an overview of the setting. A detailed one of course, but an overview....
Basic rundown on the geography, the inhabitant (review of the races, plus Grey Elves; which cultures apply, etc...), the calendar and weather, and then maybe one or two locations (Mirrhyc and/or Hyb Ciuin (this one might be a seperate product just because of the size it might turn out to be) and then a few adventure seeds type of things...