How do you write a story that is made up of a collection of letters? Like intro to Frontlines by Micheal Grant.
I suggest that you could do it like Frankenstein, where the whole novel is technically a few very long letters from the doctor to his family(?). If you wanted, however, it could be one chapter having a separate letter and ending off with "Yours", "Regards", "Sincerely", etc. You could even introduce an unreliable narrator with them not remembering details correctly, or having to write them hurriedly because they are under some outside pressure! A murder mystery, perhaps? Psychological thriller? I dunno, man, it's your book!
Use dates at the beginning of each chapter, with each chapter being a different letter, unless they are very very short then put them in the same chapter. It would be cool if it was all letters from one person to a person/people and it’s unreliable, but it’s okay because it’s real.
Or that one by Jane Austen. Umm… goes to check what it's called Love and Friendship.