Ooh I thought of another aspect of letters within fiction. A letter or any other text message is unlike dialog in that it persists to be re-read later, maybe not by the intended recipient, and can be loaded with hidden meanings that you wouldn't expect someone to pick up on or remember accurately if they heard the same words spoken.I used this twice in The Silk Mind, in a letter to the Badger Survey from Doctor Grey, and rather pointed diplomatic letters between him and Celandine.