Fantasy books/series which actually exist in their worlds

So basically, ever since I finished the Broken Earth trilogy, I've been obsessed with how talented spec fic authors can ground the books themselves in the worlds they're told from, along with all of their contents.

Reading Dune right now, and I'm loving the excerpts from texts by Princess Irulan. They make me imagine that the book itself is a historical novel by a future author who has an alternative viewpoint on how things transpired (looking at you, Yueh), and perhaps even took some creative liberties.

Of course, there's also Tolkien, who 'translated' all his books from the languages they were written in, like he did with many ancient real-life stories in his career.

Wondering if anyone here has any other examples of sf/f books having a diegetic reason to exist that stuck out to them? Was it interesting to you? Was it similar/different to the three examples I listed above? Are there examples where you think it didn't stick the landing/can take it or leave it?

And also, are there any variations on this trope that you haven't seen before and think would be interesting?