I don't think you need to worry.

The cynic in me thinks there's no worry because I am struggling to imagine who would use this. I mean, who is your target market?

Who cares enough about a book to read other people's notes, while not reading the book? High schoolers? Book Club members?

And who is going to the effort to write notes and share them publicly? People who want discussion with other people who haven't read the book?

If you were building community around those who had read a book, and wanted to discuss it; that at least makes more sense. But I'm also pretty sure I don't want to hang out with the kind of person who would love a site like that.

I get it, I read a lot, and this is a site for non-readers, so I'm not your target market.

Back to the legal question, let's assume for the moment, that you build a community. What exactly is your legal concern? Copyright? I assume you aren't posting the whole book right? Other than providing book downloads I'm not sure what legal issues you expect to encounter?