Maybe it could be a disease that everyone is infected with; it only reveals the lore in the right circumstances, but spreads benignly otherwise. Doesn't fit the OP's need for something that could be stolen, though. (Or substitute vermin that are persistent; supposedly cockroaches as a species can survive almost anything. Or a recessive gene in all descendants of the druids who created it.)
d20 Gamma World has this everywhere everywhere in the form of nanites and the occasional biotech. And weaponized. Part of the fall of civilization was rampant ideology and agenda wars taken to their logical conclusion. Afterwards there are remnants of various fringe groups that literally infect and overwrite others to force their ideals. More benign versions just upload relevant data to whomever can access it or persuade it to do so if it is not automatic. Same with equipment. Everything from toasters to grenades were fittted with artificial souls as effectively the OS and instruction book for the item. Or as security. Some had layers of souls set up that needed to be persuaded in order to learn the full potential of an object.
In Albedo an assassination attempt at a lab autopsying a Creator(human) found in a derilect destroys the quarantine chamber and nanites in the corpse, now blasted all over the lab, promptly helpfully upload their control routines and commands to Erma and the Net(a single AI running every computer) and go to work healing her injuries.
In Star Wars one of the ancient races opted to devolve themselves to escape an invasion. Eventually an event triggers a genetic code all the natives promptly re-evolve into the original race. Complete with their old knowledge.
Tommyknockers plays with this as well. The material if the alien ship ablates off as a sort of mist/virus that gradually increases and instills in the locals the aliens knowledge, while also gradually transforming them into the aliens.
Lots of others. But those come to mine right off.