Here are a few I'm particularly fond of, especially Cloud Atlas. Cloud Atlas' cover made me so interested, it's literally the only reason I bought the book. (And unlike Twilight, I was very pleasantly surprised!) I think non-illustrated covers can be well-done, classy, and suitable for the book just as well as illustrated ones. While hunting for covers, though, I did discover that nearly all my favourites are illustrated.
The Never Let Me Go one is the hardcover edition I am so giddy to own. As pretty as Keira Knightly is, she's NOT the character, so I don't want her damn face on my book! Stupid movie covers... But the point is, all of these really work for their respective books. Cloud Atlas is divided into several different viewpoints/characters, and it's called freaking Cloud Atlas, hence the separated clouds. Death in Venice just works, somehow, showing Aschenbach but not actually showing his face (which is good form, imo.), and having the world be tilted and off-kilter just like his is when he arrives in Venice. Never Let Me Go manages to capture the sort of dreamy/reminiscing feeling of the novel. Yeeeeee books. God I love books, shut me up before I go on and on.
And yeah, Twilight is unfortunately beautifully designed. Was looking for some fluff to read, thought it was particularly well-designed fluff, thought, "This should be good!" Nope. It wasn't just fluff, it was like, 99% air. You really shouldn't judge a book by its cover.