Author:Kathryn Wesley

From the "About the Author" in The 10th Kingdom:

"Kathryn Wesley is the pen name for two best-selling authors. They have won many awards, including the World Fantasy Award and the Hugo Award. Together, they have written over sixty novels and hundreds of short stories. They live and work in the magical kingdom of the Oregon Coast with seven cats."

This is pretty recognizable, and FantasticFiction verifies that this refers to the wife/husband pair of Kristine Kathryn Rusch and Dean Wesley Smith.