Elen's Island by Eloise Williams

When her parents send her to stay with a grandmother she hardly knows for the summer, Elen is furious. Gran lives on a tiny island and doesn't want her either - it's not an easy start. Gran's idea of childcare is to give Elen a map and tell her to explore. Who is the odd boy on the beach with a puffin? After saving Gran in a storm, Elen finds a picture that she's sure is a clue to hidden treasure. She investigates - and finds a very different treasure from the one she expected.
Binding: Paperback / softback