A three-part graphic novel was also announced that will follow the ending of The Legend of Korra finale, called Turf Wars. It is being written by show co-creator Michael Dante DiMartino, and illustrated by Irene Koh. The first part is due out in June 2017. You can read more about the announcement at Entertainment Weekly and the Dark Horse blog.
In other news, Korra's adventures will continue in comic book form, picking up where the tv show left off and according to Mike DiMartino (who will be writing it) will focus on Korra and Asami's relationship. Not much else is known yet, but you can read more about it and see the key art (drawn by Bryan Konietzko) at IGN. It will be published by Dark Horse and has three volumes planned.