Letters can be read by using them. Letters are numbered, suggesting there are a fixed number in each game.
The text of the letter seems to be largely randomly generated. Letters with different numbers have the same sentences.
Some of the text refers to Greek mythology (the rivers Cocytus, Styx, etc) and some sentences are from a Samuel Taylor Coleridge poel. Be pulveris'd by Demogorgon's power is from "On Donne's First Poem".