How do you find a fiction book without knowing the title?

This question was placed under the category, 'Dewey Decimal System' which is a system used by most libraries all across the country and it helps people find books. Now I know that the question is '...without knowing the title...' so the Dewey Decimal System is completely irrelevent but finding a fiction book (or any book for that matter) will be somewhat hard unless you ask someone you know who you think has read the book what the title was. Maybe explain to the person what happened in the book and they will know?