Margot & Me by Juno Dawson

Fliss's mum needs peace and quiet to recuperate from a long illness, so they both move to the countryside to live with Margot, Fliss's stern and bullying grandmother. Life on the farm is tough and life at school is even tougher, so when Fliss unearths Margot's wartime diary, she sees an opportunity to get her own back.

But Fliss soon discovers Margot's life during the evacuation was full of adventure, mystery... and even passion. What's more, she learns a terrible secret that could tear her whole family apart...