This might be odd to ask, but, what was the point of Book One?
Honestly, if you compare the impact of the events Season One had on Season Two verse Season Two's impact with Season Three, there's a big difference.
Yeah, I know that season one develops it's characters and that season two de-evolves them. But really, season one isn't needed.
All the character growth in season one was almost completely tossed away in season two.
Amon is only referenced twice, by name and later when Korra goes into that tree thing.
Book One, in the overall series, had no impact.
Amon, a great villain, made almost no impact even though he was an emotional and physical challenge to Korra.... Referenced twice, never referenced in Book Three.
Sure, some tech created during Book One is in Book Two, the planes and the mech-tanks, but those can be easily explained.
The only one you can argue about is Future Industries facing bankruptcy, but near the end of Book Two, Varrick saves it (What happened to him anyways?).
It's almost unneeded.