This is a wonderful novella. There are a number of reasons I say this: it is concise, yet it describes a vast fantasy world in clever little dashes of detail; it stands alone and is, contrary to one of the reviews, satisfying both in length and content; and though it sprawls and swoops and dazzles, this breathtaking journey is over before you know it--the read effortless.
The novella is a form that has fallen out of favour in the English-speaking literary world. The reason for this is that it denies our obsession with indulgence. Its strength is restraint, which many writers and many readers unfortunately don't value.
Campbell's strength here is that the world he draws is real and enthralling, and yet it is constructed with an economy of language not present in most novels. Campbell switches between two primary characters as he narrates this story: an angel and the man she seems fated to kill. Their stories are interwoven from the start, of course, but Campbell's skill is that the two stories diverge in the novella's middle, apparently moving away from the course we have been expecting, and then gradually, gracefully their stories converge again. This creates the narrative tension. Campbell's ending becomes more delightful as the anticipation guides us to a fulfilling and interesting resolution.