Well, lo and behold, Nickelodeon has ordered more Legend of Korra! The 26 episodes, which Entertainment Weekly is calling a second season, are in addition to the 14 episodes (Book 2, presumably?) that will air early next year.
Book 1, which concluded last month, averaged 3.8 million viewers an episode, occasionally beating such fare as Game of Thrones.
UPDATE: Co-creator Bryan Konietzko explains seasons and books.