Unboxed – Non Pratt

Four friends meet up at their old school to open the memory box they stowed there years ago – with five letters inside for four of them, because their friend Millie has died. When they open the box they find a new letter from Millie and discover that she has left them special instructions: permission to open her letter only if they all read aloud the letters they wrote to their older selves, revealing their deepest secrets.