This is already possible for Harry Potter specifically. There was a study demonstrating that Sonnet 3.7, among other models tested, could reproduce the first Harry Potter book 95.8% verbatim[1].
...only if you deliberately attempt to extract it by repeatedly prompting it to complete fragments of the book. They had to do quite a bit of work to make this happen.
Thanks for linking! I've been thinking about trying something like this myself.