I listen to my client and I think about the wants of their target audience. A good book cover is like any good advert, it should make you HAVE TO pick it up. I feel the key to any design is understanding what the audience wants, a book cover should go beyond showing a scene from the book, it should convey the essence of the story in a dramatic, emotional and aesthectically pleasing way that says, "Hey! Read Me! I'm what you're looking for."