Making cover art must be challenging. Not only does it have to draw an audience to the book, but the graphics themselves must represent both the subject and the feeling of the story. Take The Great Gatsby — the artist of this classic cover merged the romantic glamour of the upper class setting with the longing and angst of the lovelorn characters. The quirky cover of A Clockwork Orange is a fun nod to its futuristic setting and rebellious tone.
Whether you've read the books or not (and I suggest you do), these covers stand alone as art.