Here's why I think the Twilight cover is so good:
In art, black white and red is a striking color combination. When I was in college, I took some photography and one of our assignments was to create a black, white and red image.
It's simple and arresting. A black background with hands holding an apple. What does the image say?
The solid black background is mysterious and sinister. Under normal circumstances, you would see the person's body behind the hands.
In western culture, the apple is a
powerful symbol. It represents knowledge (the apple hitting Isaac Newton) and also is the forbidden fruit. An 'apple' is what caused the fall in the garden of Eden. An apple is what the witch tried to kill Snow White with. It is something that is both pretty and dangerous.
And the hands seem to be reaching out of the dark, presenting the apple to you. It is friendly and possibly a threat.
That's what I got out of the Twilight cover before I had an inkling of the plot.
And when I learned what the book was about, it seemed very fitting.
It may well be that the cover is better than the book it is representing, but doesn't that make it a good cover?