Lovecraftian horror and Celtic myths, which in my opinion are much scarier than Cthulhu and his bunch, so what could possibly go wrong? Well, you can have characters that will be grating to anyone not from a specific part of the United States. And it's funny, because as it is hard for us to comprehend the disgust and horror some things generated in Lovecraft, I am sure future people will be reading The Twisted Ones and feel the same about Ursula Vernon.
That isn't to say that the book was not good. I liked the ideas in it, I just couldn't like the people in it, especially the main character. The self deprecating humor, the hysterical laughter when something horrific was happening, the meme references, the many mechanisms she employs to self deceive herself that some things are not real, the fanatical belief in the order of the American system and that bad things couldn't happen to her is she just look the other way, all of these things felt so wrong to me. The writing style, with many repetitions of the same things, felt more compulsive than entertaining. I really want to believe that the way the main character was behaving was meant as a parody of the Lovecraftian gentleman hero, but I am afraid the author was quite serious in writing her.
If you can get past that, this is quite a terrific story. This woman comes to clean up the house of her dead grandmother, who was also a hoarder, so she spends days after disgusting days putting garbage in bags and getting rid of them, with only her dog as company. She comes upon a mysterious diary of her grandfather's, talking about eldritch things while also being incredibly opaque. Then things start happening.