I also tested this with Harry Potter books (which are still not public domain AFAIK). It starts auto-completing the correct words quickly, and then hangs and eventually stops producing output. You can call the API to generate more tokens, but it stops again after producing a few more correct words.

I think for a few high-profile authors (for books like Harry Potter and Game of Thrones), OpenAI probably installed some output filters in order to not get sued too hard. Of course, I can't definitively check that without access to the raw model. Which OpenAI conveniently doesn't provide.