In the Forests of Serre, by Patricia A. McKillip; Ace Books (an imprint of the Berkley Publishing Group), 2003. This is another one of McKillip's delightful fantasies.
RPGs Need Campaigns
I think that the most important supplement for a a newly introduced roleplaying game setting (whether part of a dedicated RPG or as a supplement for a generic RPG) is a campaign. I'm thinking of things like Warhammer Fantasy Roleplay's The Enemy Within campaign, or any of several Call of Cthulhu campaigns, or Pendragon's The Boy King. You really need a good campaign to see extended examples of what a setting can do.