The Cloud Searchers (Amulet Series #3) by Kazu Kibuishi

Kazu Kibuishi's enchanting series about two ordinary children on a life-or-death mission continues!

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