Just tested with 7000 books in the want-to-read list so passes the scalability test. Recommendations in decreasing priority:
1. Age > 75 is important, many older but super valuable books didn't show up as a result. Upto 200 years ago is good enough but we'd still miss Aristotle etc.
2. Ratings and Reviews should also have a higher or infinite upper bound, I'm not sure how the front end would look.