Avatar: The Last Airbender: The Shadow of Kyoshi (Chronicles of the Avatar Book 2)

"Kyoshi's place as the true Avatar has finally been cemented--but at a heavy cost. With her mentors gone, Kyoshi voyages to the Southern Air Temple, where she can master the elements beyond what she could have ever imagined. But while her bending grows stronger, a mysterious threat emerges from the Spirit World. To stop it, Kyoshi, Rangi, the daofei, and their reluctant allies must join forces before the four nations are destroyed irreparably"--