Sounds like the problem is that your sample size is too small. What happens if you expand the timeframe out to a month? Or a year? That should give you books that are cited repeatedly, not just the books which have recently been mentioned by high-karma HN users.

Agreed. Monthly seems like a much more natural timeframe for book recommendations than weekly. Trailing 30 days would probably be the best for the homepage. Then allow browsing by calendar month.

You can rank by upvotes and use karma as the tiebreaker.