For the Wizard of Earthsea books, there's an afterword (at least in the later editions I have around the house) where she says she just sketched out the map first even before any characters, and filled it in with the stories as she went.

(How many more stories would she have written in that world? I wonder)

Also: collection to her maps

http://www.ursulakleguin.com/Maps/Index.html

as well.