The Reckoning of Roku novel came out early in some bookstores, namely Books-A-Million, which is where I got my copy from. I read through the entire novel last night. Here's the things that caught my attention on my read-through. Spoilers, obviously.
During a fight, Roku casually incinerates some rocks that an earthbender throws at him. It's later established that firebenders can heat up rocks to the point where it turns into lava, though only earthbenders can control said lava.
There's a person who uses airbending to slice a man's ear off, and generally uses it to slice her opponents. The ttrpg already stated that air slices could be performed without a staff, so it's cool to see it see in action.
Roku enters a spiritual cavern that greatly amplifies bending power, and while he's there he can produce dragon fire, confirming that it is stronger then regular firebending.
Speaking of said cavern, there's some major blueballing concerning it as when Sozin first hears about the cavern from a scroll, the scroll's writer states that firebenders utilizing its power are capable of moving the sun. Yes, you read that right. Unfortunately, the scroll later turns out to be apocryphal, with none of the firebending displayed being close to that level; however the dubiousness of the account mostly comes from the claim that the entire island powers up benders, when the powerup is just relegated to the cave on the island. It still grants a power boost as mentioned; one of the plot points of the novel is that the miles-long island where the cave is located is shrouded in mist. The mist turns out to be created by three waterbenders who are in the cave, and its pointed out that a bending feat like that would normally require a hundred benders or so. The scroll did state that waterbenders could control the weather, which turned out to be true. In fact, the feats it states about earthbending and airbending turn out to be true as well. So there remains the possibility that the sun statement could be true. What's more, Roku directly compares the power of the cavern to what the avatar state possibly feels like, though he hadn't unlocked it by that point. It's also worth noting that none of the characters ever learn who Asho (the author of the scroll) is, and what became of them after they left the island is never revealed. So it's possible that this novel might have a sequel that ties up some loose ends, just like the previous avatar novels.