There are quite a few. But the exact number can change as new ones might be released. You can check the official publisher's website for the most up-to-date count.
Well, without having read it specifically, I can only guess. But generally in the Airbender world, it's a place full of elemental powers. So this novel might tell the story of the last airbender's struggle. It could be about their fight against the forces that are disrupting the balance in the world. They might be on a quest to find other airbenders who might still be alive, or to master all the airbending skills to become a true savior of their world.
Avatar: The Last Airbender is indeed an original story. It has its own distinct style and narrative that sets it apart. The combination of elements like bending powers, diverse characters, and complex story arcs make it a one-of-a-kind creation.