Really cool idea (and nice design). I know it was a weekend project so I bet most of my feedback are things you've already considered, but here it is:

1) It's a little hard to tell the source of the quote from just the thumbnail, unless you recognize the cover. I think there should be a title above the quote that includes Book and Author (in addition to the thumbnail of the cover)

2) It's such a cool concept, but I'd like to be able to browse by books (business books, psychology books, fiction, literature, best sellers, etc).

3) Make the page more dynamic, so you don't need to refresh the page every time you call for a new quote. This will make it much nicer/enticiing to quickly scroll through quotes. The refresh creates a bit of friction

4) Make it easier to tweet/share it out. You have a tweet/pin button in the corner, but I would integrate it into the quote box. If it's a good quote, I want to post it to other places