Warning: the following contains spoilers for Star Wars: The Last Jedi
Star Wars: The Last Jedi brings a lot to Star Wars lore, including the introduction of the ancient Jedi texts found in the Tree Library near the first Jedi Temple on Ahch-To. The books were prominently featured in the movie's first trailer and immediately caught the attention to fans since paper books haven't bee previously featured in a Star Wars movie, leading many to incorrectly speculate that the books could be the fabled Journal of the Whills.
The Journal of the Whills isn't a concept that has been featured prominently in a Star Wars movie yet, but they were at the core of George Lucas' original conception of the Star Wars saga. "A long time ago in a galaxy far, far away" is a phrase that doesn't only echo a fairy tale like story, but also indicates the movie to follow is a story that is being told. The idea behind the Whills was that they were beings that had recorded the events of this story into the Journal of the Whills and the story we were seeing on screen ultimately played out from their perspective (one rumor even posits that it was R2-D2 that passes the record of events on to them long after the end of the Skywalker saga).
The novelization for The Force Awakens opened with a passage from the Journal of the Whills, officially bringing the concept into modern canon, and the Guardians of the Whills in Rogue One marked the first occurrence of the word "Whills" in an actual Star Wars movie. While the ultimate plan for the concept isn't clear, there's an apparent push for the Whills to play into Star Wars in a big way, so the assumption when the books were shown in the first trailer for The Last Jedi was that the journals themselves were going to be featured, but that's not what happened.
Instead, the books (as could always be deduced from the Jedi logo on the cover) were actually ancient Jedi texts. We never get a peek at their actual contents, but we do discover that Luke himself never read them in their entirety. As Yoda says, "page-turners, they were not." While it was made to appear that the books were burned when Yoda destroyed the Tree Library, the old Jedi ghost says "that library contained nothing that the girl Rey does not already possess," which, in traditional Yoda fashion, meant more than met the eye, as we later discover that Rey had seized the books before leaving Ahch-To and they are now onboard The Falcon with Rey and the remainder of the Resistance.
The Jedi texts will doubtless play a role of some kind in Episode IX (even if it's not explicit), but it's not sure if the Journal of the Whills will rear its head again. If there were ever a time for them to be relevant, it seems like the conclusion of the Skywalker saga is the best place to feature the concept, but, just like how George Lucas himself relegated it to a behind the scenes concept, the Whills may remain just that, not playing a part in the larger plot of the sequel trilogy's conclusion.