Books are important repositories of lore and knowledge. They can make valuable treasures in almost any sort of game. (Except Star Wars pre-Episode VIII...)
Spellbooks have long been an important form of treasure for wizards and the like, who need to find archives of arcane instructions in order to expand their magical abilities. But books can also provide training instructions for other types of heroes. Dungeons & Dragons has magical tomes that provide benefits to various character classes, but this could also be mundane knowledge such as weapon drill routines and physical fitness exercises that could benefit someone who reads and applies the material.
Books can also provide background information that reveals interesting and perhaps relevant details of history, culture, geography, ecology, or other facts. They could also contain esoteric information about the supernatural, other worlds, or extraplanar creatures that could come in very handy for combating weird foes.
Another underrated thing that books can contain are records. Financial records, legal records, census data, and so on. This might feel a bit more dry, but can provide valuable clues or evidence needed to take down evil corporations or other entities.
Commentary by memnarch (who has not seen the movie)
Uhhhh. Huh. Well that took a sharp left turn; frankly I'm rather shocked that Star Wars actually has regular books now. I mean sure, not having paper anywhere is rather silly for a whole universe, but it's part of the whole setting aesthetic here! Not having paper means you get holograms and datasticks and stuff like that!
And I guess this means that there's no vision quest thing like in Episode V. Which, fair enough, we haven't had the time to satisfactorily set anything like that up. I might be reading too much into the parallels for this movie to the original trilogy as well. Sure, there's some events that match, but at the same time, Rogue One had a lot of events where the Rebellion getting the stuffing kicked out of them looked to be a central theme. It's hard not to try and draw those parallels though.
Anyway, now I'm really curious what the purpose of having paper books introduced is. It doesn't feel like something the people working on the film would just throw in because they looked old. Especially not with the close up shot of that starburst imagery. Though perhaps the books being paper is a comic item. I know there's a few different kinds of material that can be used for writing sheets like papyrus or vellum. Vellum even has the benefit of not requiring a lot of plants that don't seem to exist on the island, though it might be a bit much for a Jedi to want to do.
Rey: I approach the relics. Do I recognise them?
Luke: They’re books.
Luke: You’ve probably never heard tell of such artefacts before.
Luke: They’re made of paper, the forbidden material.
Rey: Why is the Force so strong around them?
Luke: Paper is made from trees. It’s full of midi-chlorians. And this paper has ancient Jedi records written on it.
Luke: Names. Families. Bloodlines spanning thousands of years.
Rey: Why are they here? Shouldn’t they be in a library or something?
Luke: Anyone could find them in a library. I did.
Rey: You stole these from a library?
Luke: Borrowed! But they can never be returned.
Rey: The information in them must stay secret?
Luke: That, and I can’t afford the overdue fees.