The Secret Place, Tana French

Detective Stephen Moran hasn't seen Holly Mackey since she was a nine-year-old witness to the events of Faithful Place. Now she's sixteen and she's shown up outside his squad room, with a photograph and a story. Even in her exclusive boarding school, in the graceful golden world that Stephen has always longed for, bad things happen and people have secrets. The previous year, Christopher Harper, from the neighbouring boys' school, was found murdered on the grounds. And today, in the Secret Place - the school noticeboard where girls can pin up their secrets anonymously - Holly found the card.