The Glass Demon by Helen Grant
Teenager Lin Fox has been dragged, along with the rest of her family, to a crumbling castle in the wild woods of Germany. Her father is taking a sabbatical year to attempt to become a full professor back in England. He hopes to find and publish on the Allerheiligen glass: a series of famous, and elusive, medieval masterworks in stained glass. Of course (given the title) the glass is rumored to be possessed by the demon Bonschariant and the curse (and the demon) seem to be active once again.
This is an interesting blend of medieval history and present day mystery. I know I was having trouble deciding if a mortal human was behind the murders or Bonschariant was on the prowl, especially since broken glass was found at each of the crime scenes.
More psychological fiction than horror this one will keep you on your toes guessing what’s happening and what or who is stalking the woods at night.