Liar's Moon — Elizabeth C. Bunce

As a pickpocket, Digger expects to spend a night in jail every now and then. But she doesn't expect to find Lord Durrel Decath there as well--or to hear he's soon to be executed for killing his wife.

Durrel once saved Digger's life, and when she goes free, she decides to use her skills as a thief, forger, and spy to investigate his case and return the favor. But each new clue only opens up more mysteries. While Durrel's marriage was one of convenience, his behavior has been more impulsive than innocent. His late wife had an illegal business on the wrong side of the civil war raging just outside the city gates. Digger keeps finding forbidden magic in places it has no reason to be.

And it doesn't help that she may be falling in love with a murderer . . .