So look at which books I finished and rated highly, find people who overlap strongly with that, and recommend the set of {their books - my books}. As I recall,

So look at which books I finished and rated highly, find people who overlap strongly with that, and recommend the set of {their books - my books}. As I recall, this is basically what won the Netflix recommendation engine contest. There's going to be a lot of computation in crunching the numbers, but it's not hard to come up with something better than "recommend what has the highest average star rating".