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I agree that it probably isn't realistic that characters just happen to find random letters lying around every so often. So make it more varied. In one chapter the characters find a book wedged in...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47869 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
I agree that it probably isn't realistic that characters just happen to find random letters lying around every so often. **So make it more varied.** In one chapter the characters find a _book_ wedged into an opening, in another they come across a _newspaper article_ someone taped to a wall, in another a _bulletin board_ with the latest schedule and events. There's more: You can have blueprints, photos, protocols, notes, handwritten notes, presentation slides, inventories, graffiti, technical drawings/diagrams, and, yes, letters. You could also relay information via non-visual means, e.g. an answering machine, a dictaphone, an automatic emergency recording.