When you last played an RPG, did you ever go to the town library and read the in-game books? Did you wonder who would spend time writing those?
As part of my work on Waking Nightmare E4, I had to make a large collection of in-character letters of no plot relevance (that the players had to go digging through in order to identify the actually relevant documents). Since I wasn’t up to the challenge of writing them all from scratch, I did some searching and found the website www.lettersofnote.com. This contains a treasure trove of correspondence, either between notable public figures, or of some other value (e.g. poignant, or humorous). Crucially, it’s far quicker to adapt an existing letter than it is to write one from a blank sheet.
Browsing through that letter collection has its own value, though – it’s nice to see the humanity in history.