(Assuming this is the M. Gira book, because the link doesn't work for me.)
Honestly, it's not really the most fucked up book I've ever read, but I've been desensitized by reading fucked up books. It's mostly been hyped up as "Dark" by normies who read "Regular" books and don't stray off into bizarro horror all that much. Because so much ot it comes from the shock, the first real "Out there" scene in a surrealist horror/tragedy you read is inevitably going to be the worst one you've ever read, and the rest of the book will be desensitization from there.
As a horror book, I suppose it does its job pretty well. Gira seems to have all the building blocks of a creepy experience, and he knows where to put them. Reading this felt a lot like my first time reading Stephen King: "Wow, I get why people think this is scary, but you have to get really invested and immersed in order for this to work, because as soon as you step back and get on a train of thought that it doesn't want you to be on, it can easily get really goofy and edgy-sounding." Because of this, I wasn't so much creeped out or thrilled by the stories so much as I was intrigued by the metaphors and amused by the edgy imagery.
From sheer content, yeah, I suppose it's among the most fucked up books I've ever read, but there's arguably more, worse shit in books like Flan, and similar Bizarro Candide-Clones. And also just about any book written by Wrath James White It depends on which book by Wrath James White... Hell, the most fucked up book I ever read was the beginning of Lolita, (Never had the chance to finish it, not really convinced that I want to.) and that was all conceptual. No smiling dogs or gang rape to be found there.
