I have an interesting theory related to the Library in episode 2-10, and the circumstances where it was used by people in the past.
One thing that has made me wonder after watching that??episode??is why the Owl Spirit (I have no idea how to spell his name) chose when he did to sink the library and try to kill the people in it. Considering the ways it had been used in the past, by people such as Zhao, considering that Fire Benders had burned a huge section of it, why did he choose then to sink it? Why had he not done so sooner? Was it just like the old saying of "the straw that broke the camel's back"? Or was it something more? Was there something in particular about what Aang and his companions wanted to do that made the Owl Spirit unhappy?
Something about the way he asked "You think you are the first to think your war is just?" gave me an idea, and it has spawned into an interesting theory. I think that the Fire Nation used the library to find out about the Comet and how it could be used. They used it to get the tools they needed to launch??a devastating strike against the other nations.??And... I think they had a very good reason for thinking their war was just.
I think that someone else, sometime beforehand, had also used information from the library. This someone or something used it??to find out about the Solar Eclipse. And then??he/it/they did something horrible to the Fire Nation. That was the so called "darkest day".
So then, the Fire Nation retaliates against the world. They too??use information from the library to do so. They also destroy any information about themselves... not out of any malice, but in a desire to protect themselves from any further attacks.
And then, much in the future, Aang and his companions learn about an upcoming Solar Eclipse, and want to use it to attack the Fire Nation. Completely and utterly unaware that someone earlier used to same information to start the chain of events that lead to the war they are trying to end.
Of course, the Owl Spirit, who did know this, would have found the whole thing tragically ironic. It must have seemed like some horrible cycle that was just going to go on and on. Hence, he finally decided to bury the library.