Well, you could create another table, called 'vote' or something similar, which holds all the votes that are cast. Then you can query that table and select all the rows for that book, then perform whatever algorithm you use to get the final score, and then output that.
If you want to restrict people from voting multiple times, then you could do this one of two ways, the first being slightly cleaner, but both having their faults (in reality, there's no simple foolproof way to prevent someone from voting online multiple times): the first being to record their IP address with the vote, and then check the vote table to see if they've already voted for a specific book; the second being to use cookies to log what they've voted for. Of course, they can change their IP address, and they can delete cookies, so there's the problem with both the methods. If you use usernames and they have to log in, then you can just prevent a username from voting multiple times.
