Escape from the Spirit World

Found here: Escape from the Spirit World

Although a game, the stories and plot details explained in the game are considered to be canonical. The game is meant to explain the events between the end of Book 2- Crossroads of Destiny and the start of Book 3- The Awakening.

After being shot by Azula's lethal lightning attack, Aang falls into a deep slumber. During this, Escape from the Spirit World explains that while Aang's mind and body were immobile, his soul was trapped in the Spirit World. To escape, he had to reconnect with his past four lives.

Aang met with the two past lives he knew: Avatar Roku and Avatar Kyoshi. A few more details about Avatar Kyoshi's legacy were revealed, such as her connection to the Dai Li. Furthermore, the two Avatars before Kyoshi, Avatar Kuruk and Avatar Yangchen were introduced.

Avatar Yangchen told Aang that his trip in the Spirit World would feel like a distant memory, though she hoped he would remember the lessons he learned. She also reveals that the Avatar spirit within Aang was injured, and now he has to rely on his friends rather than his past lives. This is contradicted by Aang contacting Avatar Roku in The Avatar and the Firelord.