Feb. 21 marked the official 15th anniversary of the premiere of “Avatar: The Last Airbender” series on Nickelodeon. The show, which won Annie, Genesis, Primetime Emmy and Peabody awards, has also spawned “The Legend of Korra,” a comics series and a live-action film as well as being recently greenlit for a live-action Netflix remake. Now “Avatar: The Last Airbender” has also inspired a young adult book series.