You also will have to handle people using your site to store other content. For example, how do you guarantee the notes are describing the book and not, say, p

You also will have to handle people using your site to store other content. For example, how do you guarantee the notes are describing the book and not, say, posting ads, discussing drug deals, posting pornography, mentioning things that are illegal in some countries (exchanging notes on books about various historical events could easily be illegal in some countries, for example)?