Books that are written like they were written by an actual character in whatever setting the book is for.

Books that are presented as actual books from a setting. Books like Hollow Knight: Wanderer's Journal, which is written like the actual journal of a character from the Hollow Knight universe, or The Imperal Infantryman's Uplifting Primer, which is written like a book given to guardsman in the Warhammer 40,000 universe. The physical book i can buy on Amazon is the same book a character from a fantasy or sci-fi setting could read. I think they're neat.