Have you heard of this fantasy book?

I'm looking for a book, but I can only remember a little bit about it. The main character was a girl, a major supporting character was a wizard, and there were spectres. I remember the spectres could take the form of humans, but you could tell the difference between humans and spectres because spectres didn't leave footprints. There was one scene in which the main female character thought the wizard was a spectre because he didn't leave footprints, but come to find out, he just "cheated" by hovering a little bit above the ground when he walks. Does anyone know what this book is?