Oh, but that's like shitty science fiction and pseudo science. I was probably referring to something like the book of mormon, the bible and the quram.

Like, start by having a mystical explanation for the world origin (don't mind using science as a base, but mystically interpreted and with a lot of symbols). Then some kind of strict law or precepts for the followers of the protagonist god or gods. Something that antagonizes with the non-believers. Then some poetic books, with very vague symbolism. Then some collection of prophecies, very subject to interpretation, and even better if the prophecies contradict each other. And that would be book 1, that can be "found" in the present by some modern day prophet. That book would serve as the basis of the "new" revelation and interpretation of the prophecies, adding more symbolism, and prophecies, in part two of the book.

That would be if you want to match 1:1 the bible, maybe I can settle in book 1, and have the fiction books to cite it as is, adding the "second part" in the story. I'm thinking in something like Dune, but with the complementary full sacred book, so you could read the book, see that a priest of sorts cites the book, and then go to the book and see if it's a verse taken out of context or it's faithful to the intention of the book, to foreshadow if the priest has his own agenda or not. You could read the book and draw your own conclusion of the meaning of things there. You could share a piece of the universe you are reading about in the book series. Or not do it and just read the book. I see it like an optional companion for a cool book series.

Some books, Dune included, cite pieces of texts in-universe, but you can't read them, only the cites. I think this could be cool.

I wonder if LLM can write something like that, in the same ancient style if instructed to. If the book series is not profitable, I can always start a cult with the source material, like Mr. Hubbard.