The Glass Republic (The Skyscraper Throne #2)

Pen Khan is burdened by a life of secrets, the biggest of which is written across her face. Following a devastatingly close encounter with a living coil of barbed wire called the Wire Mistress that almost killed her and her best friend Beth, Pen's face is strangely disfigured by dozens of crisscrossing scars, making her an outcast among her peers. The only person who understands her-and the trauma she's experienced-is her mirror sister, Parva, a twin confined to the world of London-Under-Glass.
Following Parva's abduction, Pen strikes a terrible bargain with the unforgiving Chemical Synod in exchange for the means to track her down, and she ventures into the unknown. In London-Under-Glass, appearance is everything, and Pen's peculiar scars make her a valuable ally. But some in the reflected city are doing everything they can to prevent Pen from learning the truth about the disappearance of a girl who shares her face.