H.P. Lovecraft. His stories are often part of my spooky reads for the months leading to Halloween. I first discovered it when I was a teenager, learning about the role-playing game Call of Cthulhu. But before I started playing it, I wanted to discover the source material as inspiration, so I borrowed in the library The Case of Charles Dexter Ward. The very version you see here, with the psychedelic cover. The same way I discovered other great horror stories, come to think of it. I thought Lovecraft was a lesser writer than Stoker or Stevenson, but I enjoyed the story and the atmosphere tremendously. I haven't reread it in years, but I might revisit it one day during one of the countdowns to Halloween. You can find it online for free here. It has a Jekyll and Hyde feel to it, even the title is a call back, but in spite of the influences it is very much its own thing, with notions of a vast arcane conspiracy of necromancers that can threaten human civilisation. This is horror on a grand scale. So if you haven't read it, I suggest you make time in the coming weeks.
19 hours ago