The Jedi Texts
One of the more interesting things in the Last Jedi is Luke’s collection of ancient Jedi texts. He keeps these in a tree-like temple at the top of the island of Ahch-To. It apparently has the history of the Jedi and must contain secrets or knowledge about the Light Side of the Force.
Luke is in a point in his life where he is in a pretty dark spot. His attempt to bring back the Jedi Order failed and he lost his own nephew to the Dark Side. At this point in his life, he thinks it’s best for the Jedi order to just go away for good. As bad as his attitude is towards the Jedi Order, he still is very protective of these ancient books.
Later in the film, Yoda visits him as a Force ghost and is able to use the Force to bring down lightning that destroys the temple the books are in. Luke freaks out at the loss of these ancient books, but Yoda just shrugs it off saying “Page turners they were not”.
It turns out that Rey took the books when she left the island, and they are now in the possession of her and Leia. We don’t get any clue of what is contained in these books, but it would be interesting to hear a passage or two from them in The Rise of Skywalker. It would be extremely easy to do as Leia is training Rey in the ways of the Force early in the film. She could be reading from the book as Rey listens or trains, and we could all hear what the ancient Jedi had to say about the intricacies of the Force.