In the most D&D thing which has ever happened, the world’s oldest (surviving) printed book was found by a cleric as part of a trove of mystical works in a secret room in an abandoned temple.
This is the Mogao cave shrine near Dunhuang, China, on the edge of the Taklamakan desert:
It’s complex of small shrines dug into the side of a hill (they’re artificial, not natural caves). Construction and use started around the 4th century AD. It grew to significant size, with dozens of individual shrines and sub-rooms. Around the beginning of the 11th century, a room full of books—mostly Buddhist texts but covering a wide range of other topics, including a Jewish manuscript—and other paraphernalia was walled off for reasons yet unknown. And in the 14th century, the complex fell out of use after a period of decline.
The site was rarely visited, but around the dawn of the 20th century, a Taoist monk became interested in it and worked to try to preserve it from further damage from the elements. While exploring the site, he realized that there was a walled off room, so he opened it up and discovered the so-called library cave. Among them was a complete copy of the Diamond Sutra printed with wood blocks in 868. There’s older printed material, but it’s all fragmentary. This is still the oldest known complete printed book.