Star Crossed by Elizabeth C. Bunce

When Digger, a renowned thief is shoved into a world of privilege, she finds herself in the middle of a dangerous, and secret rebellion. Digger pretends to be Celyn, a noble whose brother sold her to a church. Digger is forced to spy on her new employers, or be revealed as a thief. Digger soon realizes that every single person in the wintery castle of Bryn Shaer has a secret and a double identity. (Including her)

This is an exciting and well-paced read, and I highly recommend it.