I have been thinking some more about an idea to help people better understand their books. However, there's a chance that the information submitted by users would allow people to infer the contents of some books sufficiently such that they would not need to buy them. Would this be viewed as a copyright violation? Of course, it may turn out the other way around with the service resulting in more people buying books.
If the author voluntarily submits the text, it wouldn't be a problem imo. You could then/also require that any quote over 1 sentence (which could be safely covered by fair use) or maybe even any quote at all is simply a reference to the book itself- maybe even formalize the referencing system. If they are not copying the text, there is no copyright problem. Copyrights do not act like trade secrets or patents.
