Laurell K. Hamilton admits (either in the foreword or the afterword) that Obsidian Butterfly springs greatly out of the many letters she's received asking to know more about what makes Edward tick and where he's from. And she certainly delivers.
OB is her 9th book in the Anita Blake: Vampire Hunter series. While Anita Blake is certainly featured, there's actually little character development for her; most of the book concerns what she learns about Edward and his capacity for humanity in spite of his sociopathic nature.
The main item of growth for Anita in this novel is her realization that despite the training she's been receiving in her own nature as a Necromancer, she needs to maintain her connections as the human servant part of the triad she shares with Richard and Jean Claude, or be left open to attack.
There is an interesting emptiness to the book that matches the deserts which the book takes place in. I say this not in any manner of insult, but rather that the mood of the book, rather than being heavy and oppressive as many of the other novels in the series are, instead takes on a sharp, clear note...not so much empty, as waiting and ready for the next piece of action. Certainly, we'll all be starving for book 10 despite the new series coming out from Miss Hamilton.