Wandersky

Meet Wandersky-a paradise city in the clouds. Knowledge about the surface is forbidden by the emperor, but it doesn't stop his seventeen-year-old son, Prince Gabriel, from wondering. He doesn't know that the surface-called the Downside-is a mess of pollution and a zombie-like disease called the Sickness. Sixteen-year-old soldier Avery Sloane wants revenge on Wandersky for abandoning their people down there centuries ago, so her father, the Downside's leader, repairs an old airship to get her up to the floating city. But when Avery gets the Sickness, she fears her life is over.
It isn't until she discovers she has an immunity to the disease that her father makes her the leader of the revolution to invade Wandersky. When Prince Gabriel flies down there to escape his tyrant father, their paths cross-and they become locked in a war that will change the world forever.