Julia Vanishes (Witch's Child #1) by Catherine Egan
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. Her latest job is paying very well indeed. Julia is posing as a housemaid in the grand house of Mrs. Och, where an odd assortment of characters live and work: A disgraced professor who sends her to fetch parcels containing bullets, spiders, and poison. An aristocratic houseguest who is locked in the basement each night. And a mysterious young woman who is clearly in hiding--though from what or whom? Worse, Julia suspects that there's a connection between these people and the killer leaving a trail of bodies across the frozen city. The more she learns, the more she wants to be done with this unnatural job. To go back to the safety of her friends and fellow thieves. But Julia is entangled in a struggle between forces more powerful than she'd ever imagined. Escape will come at a terrible price.