the only info i have is basic..

a guy owns either a book store or a library and he has this young kid working for him there
it's not yaoi, but i think it may be somewhere around the horror genre?

Mononobe Koshoten Kaikitan fits perfectly, synopsis-wise, but it's not scanlated afaik. There's another, scanlated, one with a somewhat similar premise: Azuke Honya.

that's exactly the one i was looking for, thank you! 🤣