Because of the way books are usually displayed in a library and even a bookstore, many times the covers are not fully shown. Most times, books are put on a shelf so only part of the binding is shown. If you were to shop online for a book, the front cover is what you are shown upfront, however.
At a library or bookstore, I am typically drawn to popping, neon or warm colors and contrasting patterns as well as a significant size that stands out. Online, however, just by looking, I will be more drawn to the covers in a more complex way, observing the designs and typography and order in addition to the colors. The more interesting the art and fonts of the covers are would persuade me to more likely approach and pick up the books. The cover, however, matters very little as to whether I would buy the book, I would only be concerned of the story or the content between the covers.
Below are some examples of visually interesting covers that are out today in bookstores like Barnes & Noble that stand out to me based on either color, typography, contrasting, symbolism, art, or a combination of those factors.