This is great. I hope you get some sort of referral payment for linking back to books for purchase on Amazon. A site like this could inspire a lot of people to buy a lot of new books.

I recommend allowing users to pick from a set of categories to filter quotes from genres they want to highlight/ignore. For instance, I would be interested in seeing highlighted passages from fiction novels without running into a bunch of self-help or religious passages.

As a further step, this would be a perfect implementation of social media advertisements. The most popular passages serve as a good advertisement for books. If you have a little widget that shows a random book and favourited passage, with the ability to cycle through to others, any website could host this interactive advertisement and see a portion of revenues from the amazon kindle purchases.

Keep it up, you're on to something here.