Unboxed by 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. Particularly suitable for struggling, reluctant and dyslexic readers aged 13+