The Last Airbender Series

Series Overview
Water. Earth. Fire. Air. Long ago, the four clan lived together in harmony. Then, everything changed when FireClan attacked. Only the Avatar, master of all four elements could stop them. But, when the world needed him most, he vanished. A hundred years passed and Dapplestream and her brother, Fishclaw, discovered the new Avatar, an AirClan warrior named Airclaw. And although his airbending skills are great, he has a lot to learn before he's ready to save anyone. But Dapplestream believes, Airclaw can save the world.

Books
Elemental: Dapplestream and her brother find Airclaw and soon learn he's the avatar and must learn all of the elements in order to stop Burnstar, the leader of FireClan.

Water: Airclaw must find a Waterbending mentor, but has to avoid the attacks from Scorchspirit, the exiled son of Burnstar who is bent on regaining his honor.

Earth: Scorchspirit has been journeying, questioning his path, and realizing himself. Meanwhile, Airclaw finds an Earthbending mentor, a young blind she-cat.

Fire: The Avatar Quartet find a Firebending mentor from odd sources, and he has information that could change everything

The Avatar Stage: The final battle draws nearer, but Airclaw can't live with the fact that he would have to kill Burnstar to achieve peace