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!
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