This is an interesting point, if someone develops a card catalogue for a library, they aren't providing a market substitute for each one of those books. As far as I know, "fair use" law has to account whether or not the end product can serve as a substitute for the original work.
Fair use copyright law in the US allows for selling summaries of books, and those can serve as a substitute for the original work in many cases. Many students use Cliff Notes to get through their literature courses without ever doing the assigned reading.
