The Family Upstairs by Lisa Jewell

In its heyday, the Lamb family mansion in London was the scene of glamorous dinner parties and the setting for a music video featuring a pop band cavorting incongruously amid moose heads, crossed swords, and mahogany thrones. But when Libby Jones, a cheerful, conscientious young kitchen saleswoman with no known connection to the family, learns that she’s inherited it, she finds herself the new owner of a once grand, now abandoned building with a sinister past.

Twenty-five years earlier, three people in black robes were found dead of poisoning at the posh Chelsea address, arranged on the floor like cult members fulfilling a suicide pact. In a bedroom upstairs, police found a healthy baby wailing away, who was subsequently adopted and never told about her mysterious origins, which included two teenage siblings who had vanished. This is too much for Libby to process on her own, so she enlists the help of a reporter who once worked on the story to try to crack its secrets.