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.
Refer to Amazon.com's product reviews as an example. There is plenty of information available on a book or movie, submitted by users -- spoilers too. As long as there isn't verbatim material provided in whole contexts, I see no problem. People review books all the time; they even summarize its entire content.
If I misunderstood your idea's intent, let me know.
People will probably copy some material verbatim even if the terms of usage policy says otherwise. And if sufficiently many people are contributing to information associated with a book, after a while those copied parts can add up to a significant portion of the book.
But even if material isn't copied verbatim, what happens if significant portions of the book are revealed anyway? This may not even be a summary. The contributed content may end up being larger than the book itself.
if you can get enough bits of books posted to your site that publishers start to notice and want to sue you, then you would be wildly succeeding. even so, this all sounds like fair use to me.
