Purely factional information is not offered copyright protection. However, the specific contents of a book is also a fact yet copyright is still valid. That might seem like a contradiction, but the law is interpreted by humans who make judgement calls.
Use of a hash by a database to index files is likely fair use, but a torrent site aiming to promote copyright infringement is a different argument.
PS: The Harry Potter movies also can’t be transformed back into the books, but they are clearly a derivative work.
A comedy set in South America where off screen they hear someone say obliviate and they start talking to someone who can’t remember anything is still a derivative work. Fair uses may apply or it might not, but it’s not simply a question of how much of the original is reproduced, but where information comes from.
I don't think you can claim to own a plot and a hash is less than a skit or a plot point. Infinitely less in fact because whereas even a single character on a page is some small fraction of the work a hash is a nothingth of the work. It can not be used to reproduce nothing. It is data about the work.
Someone already came up with the correct analog. Its metadata like the ISBN or number of pages. Both datum are in fact derived from the actual work but nobody argues that the number of pages is a derivative of the work.
Use of a hash by a database to index files is likely fair use, but a torrent site aiming to promote copyright infringement is a different argument.
PS: The Harry Potter movies also can’t be transformed back into the books, but they are clearly a derivative work.