After a huge forest fire in the fifth book, Renn is wandering around the burnt Forest aimlessly. She starts running when she sees green in the distance, finally seeing trees. When she gets there, she sinks to her knees and thinks about how the Forest is eternal and nothing can conquer it. Looking at the world's current environmental situation, it was brilliant.
Clan law states that humans have to thank the prey and promise to use all of it, or they'd risk untold bad luck. Now consider that today, we have widespread illegal poaching and we're living in what is basically a Crapsack World.
Even better — the main problems of the modern world are directly caused by people not respecting nature. If we learn to live in harmony with nature again, not only would poaching end, but we would no longer be living in a Crapsack World! Its not so much that Humans Are Bastards or that Science Is Bad, but that humans have forgotten how to live in harmony with nature because we're no longer in direct contact with it.
A lot of Green Aesops are in this series — in Ghost Hunter, Torak sees another Forest. He hears the voice of the Forest telling him that it is all-powerful, eternal and worth fighting for.
In Outcast, Lake Axehead is reflecting what's happening today with pollution and global warming: mutated fish and droughts followed by floods.
Oath Breaker, on the other hand, teaches an Aesop about religion. At the end of the book, Fin-Kedinn tells Torak of how when he called the Northern Lights "the First Tree" in the Far North, the people there laughed at him and told him "that's not a tree, it's the fires our ancestors light to keep warm". The Otter Clan thought that the lights were not a tree but a reedbed. Each group formed beliefs based on their home, their lifestyles and their culture. When Fin-Kedinn asks Torak who's right, Torak is silent because there is no way of knowing if any of them is right. The Deep Forest Clans are fundamentalists because "they need certainty".
Soul Eater has a similar Aesop. The SoulEaters want to unite the clans under their rule because the clans have different customs — the Sea and Ice Clans are mentioned by Fin-Kedinn as an example of this diversity as they eat seals, which are hunters and would be a taboo in the Forest or the Mountains. The Mountain Clans worship the fire spirit, the Sea Clans consider it a taboo to mix the Sea with the Forest and even Forest Clans have different customs. Values Dissonance and Culture Shock are major themes of all the books after the first.
A few times during the books, there's reference to a Great Wave that devastated the coastal regions of the Forest. Reading between the lines - the European wildlife, tundra in the north, mountains in the east, sea to the west - tells us that the series is likely set in what is now Norway; the Great Wave, therefore, may be the tsunami that occurred following the Storegga Slide.
The first book makes it pretty clear that morality comes from the clan-soul. Why would this be? Well, the clan-soul is responsible for your sense of belonging. Without it, the spirit doesn't know it's place in the world - like an animal raised in isolation, or even a feral child, the spirit becomes aggressive. This is why Torak Took a Level in Jerkass after he was cast out and found out he was clanless: his sense of belonging had just been shattered, making his clan-soul more vulnerable to the soul-sickness.
The ghost of the Burnt Hill isn't any of the Soul Eaters. It's Narik, an eight-year-old kid who has been trapped there for twice his lifespan just because he didn't get the proper rites.
And coming off of that, when Narrander dies, he probably won't get to go to the heavenly reedbed either given that he lives alone in the wilderness and won't get the rites either. And even if he could, he would have to choose between eternal ghosthood, and an afterlife without his son in it.
Whatever Narrander's relationship to the Hidden People is, fairy bargains in fiction tend always to come at a price. Add to this the fact that plenty of fairy-tale bargains involve the bargaining of one's firstborn. Maybe that was what they decided to take.
The fact that in Ghost Hunter Eostra can summon Torak's father's soul, even though he devoted his life and death to rebelling against her. In the COAD world, even an I Die Free can be thwarted - you join the Soul Eaters and they own your immortal soul.
Given that Torak's father stole part of the fire opal, he has ended up in a situation where he is now morally obligated to commit suicide, for the greater good. Once Torak was an adult he probably would have done it too.
Considering that Renn's birthday is never given, it's entirely possible Seshru was pregnant with her at the time of the Great Fire. Torak's father may have unwittingly attempted to murder his son's best friend.