Book 1 was actually designed to be a self-contained story. In its earliest conception, it was intended to something a 12 episode miniseries about Avatar Korra. That was it. The series would have ended with Amon defeated and Korra getting her bending back. Then Nick extended their order to 26 episodes, paving the way for Book 2 (14 eps), so they had to write a new storyline for that as well. Then Nick extended the series for ANOTHER 26 episodes, which means Book 3 and 4 will be about 13 eps each, and possibly more connected.