The Glass Puzzle — Christine Brodien-Jones

Eleven-year-old Zoe Badger, imaginative, carefree and adventurous, lives a transient life, moving with her mother from one town to the next--except for summers, when she stays with her granddad in Tenby, Wales. But when she and her cousin Ian discover a glass puzzle that's been hidden away for decades, ancient forces are unleashed that threaten to change their safe-haven summer town in sinister ways.