I ran a campaign where the PCs were characters in a history book, brought to life.
The gimmick was that, unbeknownst to the PCs, the world basically ended when the demon lord Baphomet conquered the whole plane. His savage presence causes text in books to turn to gibberish, and only one library bastion survived. In it was a copy of The Book of Lorem, which is the textual and physical manifestation of the Word of God.
A librarian wanted to see if it was possible to save the world, and found a history book about the group who initially found Baphomet's prison and inadvertently released him. She unbound that book, and spliced a section of the text into the Book of Lorem, which technically made the events in the history book Word of God. So the PCs, despite being long dead, were alive in the text, and could, like, learn things that were known at the time but had since been lost, and they could report back to the librarian, who would devise a way to fix things.
The first half of the campaign went well. The party, with the aid of a librarian, figured out how to fight back against Baphomet when he was released, and they banished him. But when I had the group learn that they were in a book, the players lost interest in my Inception-style plan to use a copy of the Book of Lorem inside their own book to go deeper, eventually finding God Himself and doing some weird stuff to rewrite canon.