Blightborn By Chuck Wendig

Cael is on the run from the Empyrean police with his friends Rigo and Lane, as the consequences of the first book catch up with them. He is heading for the floating city of the Empyrean to rescue his sister Merelda and to find Gwennie before she is lost to him forever.

Gwennie, meanwhile, finds herself on board one of the floating cities and learns that the high life is not necessarily a good life. The second book of Chuck Wendig's young adult Heartland trilogy, a dystopian story of farmland and sky cities.