Enchanted Isle

This story had a delightful vintage feel to it, since it's set after WW2 in England. At first it's just a girl and her explorations; then she begins to uncover hints of a darker past to the place, facts that keep shoving the past into the present and dark secrets that someone is ready to do what it takes to keep hidden. It's a bit more of a cozy mystery because that facet doesn't pop up until later in the novel and then leads to a serious showdown.

There's a dual romance to sweeten things up a bit, also!