The Serpent Sea (The Books of the Raksura)

Moon, once a solitary wanderer, has become consort to Jade, sister queen of the Indigo Cloud court. Together, they travel with their people on a pair of flying ships in hopes of finding a new home for their colony. Moon finally feels like he's found a tribe where he belongs. But when the travelers reach the ancestral home of Indigo Cloud, shrouded within the trunk of a mountain-sized tree, they discover a blight infecting its core. Nearby they find the remains of the invaders who may be responsible, as well as evidence of a devastating theft. This discovery sends Moon and the hunters of Indigo Cloud on a quest for the heartstone of the tree - a quest that will lead them far away, across the Serpent Sea. In this followup to The Cloud Roads, Martha Wells returns with a world-spanning odyssey, a mystery that only provokes more questions - and the adventure of a lifetime.