> Author might be right, but that is essentially an un-provable statement and doesn't really amount to a solution. But rather a statement that it can't be solved.

Not at all. Someone more motivated than the author can choose to decode it. If much of it was lifted, then you can trace back the lifted "recipe" to the earlier work and construct the index.

However, it's going to be very painful, slow work. For no real gain (who really cares about a sloppily done Medieval health self-help book?).