|Well, that was easy.|
1. Reliefs or Murals. Lovecraft presented horrible, unbearable facts about the history of the planet this way in At The Mountain of Madness. Rather than shout "HEY YOU KNOW HUMANITY IS A FAILED SCIENCE FAIR PROJECT" far better to describe or sketch cryptic scenes and give the players wiggle room to misinterpret them. Also keep in mind that images are a common way to present propaganda - that narrative may very well be unreliable, although its visual reality will tend to fool people seeing it more so than words would.
2. Exam Questions. I did this recently for my Game of Iron campaign- they found a suite of rooms with a bunch of unmarked exam papers dealing with the Big Questions of the dwarven past, sans question, scattered around (these being ancient dwarves the answers were graven in runes on copper foil strips). I used four different fonts to create four different students with different styles of writing and degrees of knowledge, and pulled the resulting strips of paper out of a bag.This is one of the best ways I know to impart information while creating doubt and variously reliable narrators.
3. Rosetta Cryptogram. The players have to figure out a text that's written in a known language but unknown script, with just a few clues (known places on a map, captioned pictures of known gods or monsters). This is an actual puzzle,in English or whatever language they know, for the players. Make sure to leave a few letters that can be figured out only by context ("... and he shall be freed from the ...cage? cake? cave?")
4. Shadow Play. The shadows, ghosts, or bones of persons past still enact a dumb-show of the terrible events that went on in this place. Most suited to local history but, to convey cosmic truths, you can set up a whole stage where the spectral troupe enacts a mystery play.
5. Degenerate Chant. As done well in Riddley Walker, awfully in the old Star Trek episode The Omega Glory, and with ample historical precedent ... the clue to history lies in the correct interpretation of a children's rhyme, folk song, or ritual chant, which has become corrupted, streamlined, or sanitized over time.
6. Pop Quiz. Here, it's the sage or oracle who asks the players what they think happened. In the straight-up version, all they get is a "yes", "no" or "kind of"reply to their guess, with no follow-up questions. In the postmodern version, if you're OK with the players helping create the world, their answer becomes canonical truth, perhaps with one deviation ("the sage says, 'That's almost right...'") The important thing when running the postmodern version is notto let the players know this ahead of time.