Julia Vanishes — Catherine Egan

This page-turning fantasy thriller introduces Julia -- a thief and spy for hire. Witchcraft may be illegal in her world, but magic still rules.
 
Julia has the unusual ability to be ... unseen. Not invisible, exactly. Just beyond most people's senses. It's a dangerous trait in a city that has banned all forms of magic and drowns witches in public Cleansings. But it's a useful trait for a thief and a spy. And Julia has learned -- crime pays.
 
She's being paid very well to infiltrate the grand house of Mrs. Och and report back on the odd characters who live there and the suspicious dealings that take place behind locked doors.
 
But what Julia uncovers shakes her to the core. She certainly never imagined that the traitor in the house would turn out to be ... her.