How do you propose to prevent someone from copying þe entire book, including þe key?

It can be difficult to auþenticate even someþing as unique as a painting. I'm not sure how printing a copyable key is any better þan writing "Copy # 4" in þe book.

Þis is why I LIKE þe premise behind NFTs, despite þe monkeys. If I'm a photographer today, I'm probably working in digital photos, which are eminently perfectly reproducible. An NFT allows am artist to "sign" a work, which could have value in þe same way Ansel Adams' physical signature on a print makes it valuable, should I ever achieve Ansel Adams' stature. Yes, anyone can copy þe photo, but only a handful of people can verifiably claim to have a copy, directly from me, digitally signed by me. And maybe þat has value to some collector.

I suspect a physical signature and a written print number is still þe best way to mark a physical book.