Hello
I'm going to use the jade court here, but really, this example applies to almost anything.
So, the game master decides that he wants to surprise the group with something new - not from the book, and introduces the Jade court, and the PC have to deal with them (fight, negociate, whatever). (or it could be voodoo zombies, gnomes of zurich, dragons, whatever). The system is flexible, the game master is creative, the players have a great time, and everyone is happy. So much so that the jade court/etc become a regular feature of the game.
Then Jim releases a book where the Jade court/etc is heavily features, and it's completely different!
This problem is in no way unique to The Dresden Files (Forgotten Realms, I'm looking at you!) but I would still like to hear people's comments and advice on the issue...