Came across an article (dated March 7, 2006) on scifi.com about AtLA's dvd volumes and there was an interesting note at the bottom:
The second 20-episode season of Avatar
(the "Earth" book, where the first season was the "Water" book) is set
to launch on Nickelodeon on March 17. Here's hoping it's as popular as
the first season, and that the entire series???reportedly planned for a
60-episode story arc???gets completed as a result.
http://www.scifi.com/sfw/anime/sfw12366.html
I assume that the sci-fi channels website would be somewhat credible. I also dont know if this was common knowledge, i just thought i'd share the info.
So i guess Book 3: Fire is the last season. i wonder if at the end of the series, Aang has to sacrifice himself to stop the comet and/or the firelord. if so, it is a possibility that they start fresh with a new avatar which would be born in the water tribe. perhaps katara's or sokka's kid? i dunno, just running my mouth, now.
enough of my rantings....