Pamela Dean has given an interview at the Dusty Shelf.

I decided I wanted to write a book that would have the flavor I liked in those books, the fantastical setting, the mystery, the humor, and the huge sense of gigantic forces moving in the background, only imperfectly sensed by the characters, the reader, and maybe even the author. I also wanted it to have that quality that C.S. Lewis calls "joy." He describes this quality perfectly in his autobiography. He thinks it's connected to the human yearning for and partial recognition of God. I'm an atheist, but I recognize the numinous just the same.

In any case, I wanted the book I planned to write to have that flavor for ten-year-olds AND for thirty-year-olds AND for 90-year-olds. I wanted that flavor to remain through endless rereadings. I wanted to write the most rereadable book imaginable.