This is a use case of IPFS I think. Any IPFS "website" will remain available at its original IPFS content ID as long as at least one person is seeding it, so if

This is a use case of IPFS I think. Any IPFS "website" will remain available at its original IPFS content ID as long as at least one person is seeding it, so if a book referenced one the author could seed it to guarantee that the URL will continue to work as long as they're willing to keep it up (and any interested 3rd party could do the same)