What an interesting, well written book. This is not an overtly Christian book, but I think some of the themes in it could be used as a jumping off point for a very interesting discussion.
Certainly there is love and sacrifice, but there is also the idea of identity and how our inner and outer selves affect one another. There is also the idea of taking the risk to break out of the safe, but stunted place we are in to participate in a world full of beauty and danger and the richness of emotion and relationships.
In the book Renee's love for Sarah causes her to take Sarah's place and to make a sacrifice that allows Sarah to, in a sense, cover herself with Renee's identity and so become more fully all that Sarah was meant to be and to take that risk and move into life.