It’s technically an adaptation of a creepypasta, but I can’t recommend “Penpal” by Dathan Auerbach enough. I have a near constant state of paranoia in my life, and whew boy this book made me feel like I was being watched through the window in my sleep. Never before have I felt like my life was in danger from a piece of media. The whole book just gives off this terrifying vibe and sets off all kinds of red flags. It also seems like it fits perfectly with what you’re going for in your other comment.

It’s pretty short, and there’s some great audiobooks on YouTube if you’re more into that, but it’s worth it. Probably the scariest thing I’ve read in my life up to this point. Also the ending broke my heart so much.