Horror books about the paranormal, stalkers, and slashers

I'd love recommendations. I really like the concept of Slenderman/The Operator, this unknowable thing with powers beyond comprehension and intentions beyond understanding. I also really like the idea of stalker and/or slasher characters, especially being in their POV, but something like Lights Out (which has been recommended to me irl) absolutely does not do it for me. All horror elements, including gore, are fine with me!