I recently bought a couple of novels with rather nice covers: Ballard's Empire of the Sun and Susan Fletcher's Eve Green.
They're excellent examples of the effectiveness of black-and-white illustration, in each case highlighted with a patch of metallic ink.
What I didn't know until I got the books and looked at the back is that they're part of a series of 15 reissues by Harper Perennial UK, all with covers by Petra Borner. What's more the 15 covers, when placed side-by-side, create one large illustration. You'll need to click for a proper look.
This is a great idea, and a good way to unify a series that otherwise seems fairly arbitrary in terms of the books included.
Here are the spines, showing the metallic inks to better effect.
Petra Borner has done a number of other lovely book covers, shown at her portfolio linked to above. I can't display them all here, but I couldn't resist those she did for the recent Penguin Poets series. Again, click for a bigger version.