Broken Wings by L. Stoddard Hancock

After an ancient war that left the outside air toxic, the shielded city of Utopia is the only safe place left in the world. Or so its citizens are taught to believe. Many dismiss the lies fed to them by a familial line of crooked presidents and choose to live outside the city's walls. But when a new president takes control, he wages a war on these Outsiders, and their leader's daughter Deryn Leon is caught in the crossfire. Five years later Deryn is a slave. Beaten and starved, she has little will to go on. But when a chance encounter leaves her with a means for escape, the forgotten fire inside of her ignites once more and she takes the opportunity with one swipe of a knife to her owner's throat. But escaping Utopia isn't easy. While hiding in plain sight, Deryn is discovered by Xander Ruby, the Utopian Guardian inadvertently responsible for her capture in the first place. Xander, who is guilt-stricken by everything he has done as a Guardian, wants nothing more than to get Deryn home. He takes her in and they soon form a powerful bond that will not be broken, even when the reality of this cruel and beautiful world threatens to tear them apart.