The Rise of Kyoshi (Chronicles of the Avatar Book 1) by F.C. Yee

The Rise of Kyoshi, written by F.C. Yee in consultation with Avatar co-creator Michael Dante DiMartino, is the first novel in the Chronicles of the Avatar series, detailing the origin story of the longest-living Avatar in history. The book is a USA Today and New York Times bestseller that has been praised for expanding the lore of the Avatar universe with complex characters and mature themes.