Even with OCR, couldn’t your copy at least in theory be laced with strategically placed minor word changes? Say throughout the book you pick 30 spots to change a word without changing the meaning of the text, or you introduce a typo. If every copy gets a different set of those that would be a unique identifier.
I think I have heard that being done with imperceptable changes in films sent for showings in theaters.
you can physically wire A into C, it’s the same protocol. This won’t be broken like other adapters because neither device even knows about it