Love, love, love this book!
Overall I loved it, but three things shine out:
1.) There were a few moments in the story that honestly surprised me. Not telling what they are, but lets just say Cassie rocks, and the author managed to leave me guessing till the very end - and we're talking about fairy tale territory here, where the road map is usually pretty well, uh, well mapped.
2.) She mixes science and magic together flawlessly. That, my friend, is REALLY hard to pull off. I was laughing so hard when Cassie as trying to consult her GPS "one mile north of the North Pole." Myths and environmental-biology are mixed together in a delicious combination of the utter fantastic of other worlds and the beautiful majestic of the wonders of the natural world. Cassie doesn't accept everything right away, but neither does she repeat 'that's impossible' ad nauseum - just the right mix of disbelief and acceptance.
3.) Cassie refuses to be a damsel in distress, and her use of taking the third option made me almost cheer out loud.
Besides a wonderful fairy tale retelling, this book also prompts thought and discussion on land preservation, endangered species, environmentalism, science, ancient myths, pro-choice vs pro-life, the afterlife, and the length of true love.
A few quibbles - I think the mom and her background should have been developed a little more. Also, the owl should have explained more about soul recycling. And the characters who were native American vs the characters who weren't and use of Arctic language is a real damned if you do damned if you don't situation.