One of the things I like are the tantalizing pieces of what our world is like -- or, at least, the world that most closely corresponds to ours in the book. Usually, what one gets in this sort of work is our world in our time juxtaposed with a rich fantasy landscape.
In the Keys of the Kindgom books, the world is probably ours, a few years or decades from now. There are all these tantalizing hints about how the world has changed, including a reference to papers. These are papers that a doctor is looking at. They glow green. She taps them with her stylus. We've got a primary secondary world that feels near-future sf in the middle of the fantasy cosmos. And Nix knows it and teases us with it. I love it.