The Cloud Searchers (Amulet, Book Three)

Emily, Navin, and their crew of resistance fighters charter an airship and set off in search of the lost city of Cielis, an island high above the clouds. The mysterious Leon Redbeard is their guide, and there's a surprising new addition to the crew: Trellis. But is he ally or enemy? And will Emily ever be able to trust the voice of the Amulet?