From publisher blurb:
Black Books: Tomes of the Outer Dark is an OSR investigative horror role-playing game where the player charactersare ordinary people (historians, detectives, scholars, artists etc.) drawn into a dark and horrific version of our world - the world of the Cthulhu Mythos. As the investigators learn more of the true horrors of the world and the irrelevance of humanity, their sanity inevitably withers away. The rules are biased towards playing in the 1920’s although there is no reason these rules can’t be adapted to other time periods. The rules are familiar - they use a D20 for most actions - and the games is class based, with 5 investigator classes - Charmer, Enforcer, Sneak, Scholar and Spiritualist. If you like old school gaming and you like horror, this might well be the game you're looking for!