Athenaeum Arcane Letters

From the intro:

The items contained in this PDF are letters from various periods in a generic fantasy setting and are intended to spice up treasure hordes, spur player characters toward their next quest, or simply provide a measure of verisimilitude to the GM’s world.
Each entry includes a physical description, the entire letter, and any relevant background information and notes. Use these letters as is, or let them goad your imagination to create new ones.