The Caster Chronicles by Kami Garcia and Margaret Stohl
I think it's safe to say that this series is a bit of a blogger-favorite when it comes to the covers. There's just something mystifying and beautiful to the black-and-white shots paired with some gorgeous text. (If you want to completely nerd-out on the titles with me, look how the first word on all four of the main books is made completely differently even though it's the same word. It's too awesome for words.) As a whole, I think it's a wonderfully designed set I'd love to own.