Avatar: the Last Airbender

"Avatar Aang travels to the spirit world to parley with an ancient power, bringing Fire Lord Zuko ever closer to discovering the truth about his mother's fate and his own past. Yet Zuko's sister Azula is becoming increasingly dangerous, threatening to ruin everything that Zuko, Aang, Katara, and Sokka have struggled to achieve on their search!" -- from publisher's web site.