I am honestly surprised by how many people see it as a masterpiece, because I think it is a horrible book. Initially presented as a mystery, it very quickly gives answers to all the riddles and becomes more like Terra Formars. And it shows.
The story revolves around a new world, born of disasters of the past. Japan, with only 60000 people left, is transformed into a dystopian country. People live inside the Holy Barriers and hide from outside creatures. Although the humans changed too - they now wield cantus, something like the enhanced imagination, allowing them to manipualte the elements around them.
The main character is Saki Watanabe (a girl, although I did not realise it for 1/4 of the book) and some other kids who study to use their cantus properly. Their world is full of mysteries and the kids learn that their society is very cruel and hideous. Just like the quuerats (hello, skaven!), who swarm the lands across Japan.
What`s good in this book if anything? The beginning. First 20% of the text constitutes a reasonable mystery with two questions: where do people disappear and who/what lives in the sheds in the school`s courtyard. Also, the characters are very well-written. Although some of the comparisons are difficult to follow, like the Lotus Farm metaphor.
It all however is bleak in the face of dozens of problems with the plot. The lexicon is largerly unexplained throughout the text. Why are tainted cats kept where they are? Why did no one tell kids what becoming adult looks like? They might hurt themselves! Why is the book full of things like 'if only we hadn`t done A, the B would never happen'. It spoils the ending and major plot twists so it`s really useless.
Sexualizing is another great problem of the book. Stop sexualising kids for god`s sake! I understand that all yaoi girls are ecstatic about two 12-14 year olds giving each other a deep blowjob. It`s gross and the reasons behind it are... stupid and unexplained. That being said, since the population of Kamisu is scarce, why force kids to focus on homosexual relatioship in the teenage years and forbid it afterwards? How exactly does it help the colony to reproduce? Stress relief? Pfft, please! There were more fiends in this timespan than ever. Why keep the girls virgins? One character loses it and... nothing happens. It`s never ever mentioned again.
Weird deaths - Subaru (Shun`s dog) died a brave but meaningless deaths. Its killer got evaporized a second later. Could Shun save his dog? Yes. Did he do it? No. And the MC`s special someone (no spoilers)? She died and her parents just did nothing? They consesnted to kill their child? This is utterly stupid.
Remember the tainted cats? In the novel they kill no one. We see them four times, all of these times they don`t manage to even wound anyone. Shun kills his, the last fiend kills 12 of them without a scratch, MC kills another one... everyone is alive and well. Why keep them in the first place then?
Tomiko says that there was no hypnosis on MC and her friends. Why do they forget things then?
And the last, which leads to the biggest problem. Queerats invent bullets and bombs. There is a division in Health Department dedicated to preventing it. Do you think they notice this? Ha-ha, and ha-ha again, they don`t. And this leads to a major question: why are queerats allowed to the Holy Barrier? If they weren`t, there would be no disasters whatsoever. But they are despite the fact that all adults say that they are dangerous.
After all this rant the conclusion is simple: the book is just a more elaborate way of bringing the Terra Formars scenario to the reader. The same fight tricks are even used: A, being the strongest, has to say that B is even better. Then B dies a stupid death. Shisei mentions that the oldest woman is the srongest. She has 360 vision, can regenerate and managed to live for almost 275 years. Then she dies because... a stone hit her in the head.
TLDR: don`t waste your time reading this book, the worldbuilding is nonexistent and it looks like there was no editing to this at all.