Greengirl — Veronica Black

When Susan takes her niece, Jill, away to the country for a week, she has no idea that they are both entering a world where time has stood still, where the villagers perform rites lost in the mists of antiquity. With the help of Stephen, a writer, Susan delves into the secrets of the past, working against time to rescue Jill from the evil circle that binds her to the old cottage and holds her to a recent grief.