In “The Last Jedi,” we reunite Luke Skywalker after he went into exile following the fall from grace of his nephew and student Ben Solo, who has turned to the dark side as Kylo Ren. When we catch up with the Jedi master, he wants nothing to do with the Force, obsessing over his failures in attempting to bring back the Jedi Order. However, Luke still holds onto one relic from his past, keeping a handful of ancient books. In fact, he gets rather panicked when he thinks they were destroyed in a fire. It is these books that hold the legacy and the teachings of the Jedi Order, the last hope of keeping their religion alive.
The movies never really show us Luke trying to rebuild the Jedi Order, save for that extremely short flashback of him training Ben Solo, though the comics and novels have him travel all around the galaxy looking for ancient sites and ancient knowledge, trying to bring back as much as he could from the Jedi. By the time of “The Last Jedi,” the only things Luke has to show for it is what he calls the sacred Jedi texts, a collection of old, leather-bound books.
They don’t seem like much, but just like the flash drive with the plans for the Death Star, Luke’s sacred Jedi texts also have a long, long history behind them.
Source : https://www.slashfilm.com/1284491/star-wars-jedi-survivor-explains-key-element-last-jedi/