aETERNAL, by Cynthia Leitich Smith (Candlewick 2009). Zachary is a neophyte guardian angel. Miranda is his second, err, charge. In love with her, he breaks the rules to try to save her, which results in catastrophe: she's turned into a vampire and, worse, into the hand-chosen "daughter" of the current bearer of the mantle of Dracul.
Cast from heaven, stripped of his angelic powers, Zachary is forced to wander the earth until he's given one final chance at redemption. Which leads him into the service of his once-beloved in the castle of the current incarnation of Dracula himself...
Can he save himself and her, or did his choice cost them both life eternal?
A striking and brilliantly affecting treatment of sacrifice, redemption, and the struggle of good vs. evil. Highly recommended.
And, yeah, I'm married to the author :-). Also available from Walker UK and Australia.