This is an unusual and gripping book that is a mystery, but is so much more. Even though it is YA, it is smart and I never felt that the book was dumbed down for a teen audience. The characters are original, and unusual. I read it in one sitting, and I didn't mind the cliffhanger because the journey to that point was so enjoyable. Mitchard is a skilled storyteller. The use of science (the characters' medical condition)is original, and works well. (It reminds me of her novel Second Nature, which also featured an unusual character with unique challenges and medical problems.) The complicated premise only adds to the enjoyment of the book. When can I get the next one?