[Kanji_The_Wanderer]

As far as I know the show is over. Book 3 ended at episode 21, making the show run for 61 episodes. The ending was satisfactory, the entire show builds you up for the final fight, and it was not disappointing.

If there was an Air Book, then I think it should be a prequel to the first one; The Air Nomads battles with the Fire Lord, while the Avatar was in suspended animation.

Perhaps start it with the Avatar's birth, then take it from there.

I think this would be the best storyline for a fourth book, since I cannot see anything else they could add to the story after it ended without making it look like crap.