You shouldn't judge a book by its cover (but we all do), yet you should judge a cover by its art!
Since a book cover is an instantaneous advertisement for a book, and we all know advertising's sole purpose is to sell a product, then wouldn't you expect all books to have eye-catching, if not beautiful, covers?
One thing I know for sure, the design of the cover should in some way reflect what you will find within the book itself. I'll bet we've all read a book where there's a gorgeous girl on the cover, only to find that the main character has severe acne and wire braces on her teeth. She may be beautiful, but not in the way she's portrayed on the cover!
Periodically, websites will feature book covers. Shortlist had "50 Coolest," Flavorwire showed us "20 Most Iconic."
The most surprising book cover I've come across is the cover for Mr. Penumbra's 24-Hour Bookstore by Robin Sloan [F SLO]. The book was recommended to me by a friend, but the cover was never mentioned. Imagine how freaked out I was to discover that, with the lights out, the book on my nightstand glowed!
If you're interested in the topic, may I suggest checking out Pinterest. Here are two boards that are a delight for the vintage book art lover: Book Bindings and Vintage Book Covers. There are many, many more!