Boundary — Mary Victoria Johnson

Never try to open the locked doors. Never question what you are told. And never attempt to cross the Boundary. Kept inside an eerie estate for their entire lives, Penny and her five friends have had no contact with the outside world, with their only windows being a handful of books, a sinister master and a secretive housekeeper. Fed up of living a mystery, Penny begins breaking all the rules and finally tries to escape; but what she uncovers is far more devastating than she could have imagined. It rapidly becomes a deadly struggle for the truth in a world where nothing is what it seems, and friendship can be either your greatest weapon...or your biggest liability.