If they picked 32 words in a book and gave them an alternative word to swap with they can uniquely identify about 4 billion books. That's not much work/not very

If they picked 32 words in a book and gave them an alternative word to swap with they can uniquely identify about 4 billion books. That's not much work/not very difficult to uniquely identify and control for quality. Of course the only payoff is that pirates now need 2+ copies of the book so that they can't be identified. You can push it harder and identify more words to get the pirate to buy more books but the return (in books bought) becomes much much worse and identifying them becomes more infeasible.