tying this data to a login instead of a pure cookie model like hn seems dangerous.

This feature doesn't require a login; you just put in the book/author you love, and it returns matches.

If you want to vote, you'll need to log in. We needed substantial protection around the votes, and this was the easiest way to do so.

Would you want a magic link that sends you a url to vote to your email or something like that?