Firesong (Brightstorm Twins, Bk. 3)

Arthur and Maudie Brightstorm are back home in Lontown after their exhilarating—and secret—adventures in Erythea. But tensions are brewing. The existence of super-sapient creatures on other continents threatens how some Explorers see themselves in the natural order of the world. When Arthur and Maudie begin hearing a strange song summoning them north—a song only they can hear—they know there's an adventure waiting for them that may change the lives of everyone in the Wide.