...Despite the show being all about the four elements. Yeah, it's totally irrelevant.

Except book 2 has nothing to do with mastering the elements, nor does book 3. Thats why book 1 was called Air.

Kuroneko / WinyPit was right, the names wouldn't be relevant. Each title of the Avatar books deal with what the Avatar is learning at the time. Book One: Water brings Aang to the water tribes as he starts his waterbending training. Book Two: Earth deals with Aang's journey to the earth kingdom to learn earthbending. Book Three: Fire brings Aang to the Fire Nation where he learns firebending. Book One: Air shows Korra's training in airbending. Book Two: Spirits delves deeper into the Spirit World where Korra learns to gain mastery over the spirits. Book Three: Change will, as the article states, deal with the changing of the world after Korra's battle with Vaatu/Unalaq. To bring back old titles would be redundant and, as established, irrelevant.