The Amulet's Curse by Ayn O'Reilly Walters
A year after her discovery of magic trees in their local park, Isabel and best friend Jess are on a school excursion to Kew Gardens. They come across Kew Palace and, knowing nothing about its history, decide to sneak into the library. A painting on the wall in the library catches Isabel's eye and she realises the man in the painting is wearing the amulet. When they head for home to tell Isabel's brother, William, Jess disappears! Isabel must find Jess but who is the man wearing the amulet? The Amulet's Curse is the second adventure for the Pritchard children in the hugely successful Between the Trees series.