There are 129,864,880 books. That's ~ 2^27. There are 2^256 seed phrases. What you're proposing reduces entropy by a factor of 2^229, at least in theory, which is like reducing a seed phrase with 24 word seed phrase to one with 2.45 significant words (I.e. not counting the checksum). I'm not counting editions, but if the average book has 8 editions, that's only 3 bits more, so 2.72 words.

In practice, an attacker would need a digital library of all editions of all books. But your book is likely to come from a smaller set, e.g. it's likely in English and less than 100 years old.

This technique is called a one time pad and is similar to seedXOR.

Really clever, thanks!

Is my assumtion right that you should take the encrypted file to an offline device (e.g. Tails) to encrypt it there and use the seed from there? I would not encrypt the seed on a device connected to the internet.

That's right, the most I can manage is having them put the seed in a wallet app and destroy it for safety, I'll be keeping the backups.

You only need the first 4 letters of each word in the seed phrase.

Security by obscurity is bad practice.

Kerckhoffs principle is rule number 1 in cryptography. There is kind of no rule as important as this one.

The secret you share would be potentially visible with others right?

Seems interesting but I'm not sure about this, can it be self-hosted?

And how would you go for setting a title that only me and them can find?

I'd prefer to not have it stored for too long on somebody else's servers, might be good for self-hosting though.