A brief story summary:
Ling PingAn is a well read NEET in charge of his own bookstore with no money problems at all becouse of fantasy gov. subsides, at day he has no clients at all, but at night when a strange fog sets in, strange cosplayers and weirdos (with no money) come in and buy books from him in exchange of rare objects or even food, although he's a bit vexed that he's not making any money, he doesn't really need it anyway and the weird objects are usually high quality goods that he has never seen, so he takes in stride.
This is how he sees things.
And now this is how his clients see things.
In their respective post-apocalyptic words, there's no hope for man to survive, every day is a miserable battle against hopelessness becouse Diablos (I imagine they meant "Devils") have taken over the world and humans have become an endangered species, less than cattle. When roaming that desolate landscape they encountered a mysterious fog that nulified their supernatural powers, in fear they approached the to source of light and encountered a dinky bookstore. Inside, they were faced with all kinds of horrors and an out of place "man" behind the counter, an otherwise normal body with deep fog surrounding his head and bright light where his eyes should be (think Kurogiri from MHA), this man offered them forbidden knowledge in exchange for the corresponding price, what they understood for a corresponding price was of course their lifespam, their soul or their sanity. In a desperate gambit they offer all the values that they had on them at the time and, unexpectedly, he accepted the trade. With this new knowledge in hand they seek to change the fate of mankind in their respective worlds and if posible, seek help again from this terrifying new "Ally".
TLDR: Lovecraftian horror gives away forbidden knowledge at a bargain
As for my review, its well made but it sometimes stretches too long, but its not really a problem to me. Its also a bit nationalistic? But not really racist, it is however a very idealized fantasy Chinese empire where a returner came back and made all the right choices to make it the greatest nation ever made, but its not the tipical malicious jingoism where they shit on every country, its a goverment that's basically perfect with little to no corruption that solves everything they set out to do and its very honorable and everyone likes them and wants to be like them, basically China is a huge Mary Sue, but at the same time, its a somewhat pure and innocent kind of Mary Sue, something like the 60's Adam West Batman, but not as charming.
Anyway, 5 stars, hope this gets picked.