A book where an investigator or group of people are looking into something creepy or weird, but without ghosts or demons please

Hi, this may be too specific of a request.

I'm looking for a book where an investigator or group of people is asked to investigate something strange, creepy, or weird. I would prefer something without ghosts or demons though.

I've already read the Lovecraftian series by Jonathan L. Howard btw.

Thanks ahead of time!