5 stars for the story, 1 for the format. Too many of the letters were the book-author talking to the book-reader, not character to character. The entire thing would have worked better - 'I had to read it as' to get through it - journal entries with a framing story. Perhaps they give their journals to their son for a birthday, and he edits then together chronologically for their anniversary, a la How I Met Your Mother. The letter format just doesn't work for scenes where the letter writer and letter reader are in the time-space. It barely works - the level of detail makes it vaguely problematic - when they're apart.
Barely got through it