Fear of loss. 
 I've read a lot of great books about the fear of loss, but none so pure and heartwrenching and clear as this. Of course, I'm biased because I love fantasy and myth and outsider kids and all tales that aren't afraid to GO THERE with the really difficult stuff. But even so, I still think this was a wonderful book. :) 
 Of course, this book leaves the fantasy interpretation open to the reader to discard as they wish, so we're open to plenty of interesting interpretations. But more than anything, it's the grief and the expectation of loss and the rage that got to me. 
 Secondly, I think it was the storytelling within the story. I'm a sucker for nested tales. 
 But the end? It goes there. Hard. 
 I'm so glad I finally read this. :)