top fantasy: #1 urban vampires #2 harry potter
top scifi: #1 star wars book #2 star trek book
recommended for you: author(s) you bought before
They're not going to do better than that. The author is almost the only thing that matters in determining whether or not you'll like a book.
Now, if it's an author you bought before and then you hated the book, there's room for improvement in the recommendation.
I can give you a few examples where there's room for improvement.
If you liked jules verne, maybe you'll like h.g. wells
If you liked isaac asimov, maybe arthur c clarke.
If you liked william gibson, maybe bruce sterling.
If you liked twilight, arrgh!
They're not going to do better than that. The author is almost the only thing that matters in determining whether or not you'll like a book.
Now, if it's an author you bought before and then you hated the book, there's room for improvement in the recommendation.