Definitely agree with Gideon the Ninth! If I knew that book would be a whodunit/magical competition in what feels like a gothic house I would never have picked it up. That's just not something I like.

Also You Let Me In by Camilla Bruce, specifically bc auf the translated versions cover (& summary) I first picked up. I went into it expecting a YA thriller with some fantasy elements and got a truly disturbing book. It's a shame, cause that book would probably have been praised more if people didn't have false expectations...