I don't usually write reviews, but I promised I would do so, and I REALLY liked this book.
In my opinion, it was very well constructed. The prose is quite pleasant to read, formal without being stuffy, and it flows surprisingly well. The story is interesting and the characters are captivating.
The treatment of the young characters is one of the highest points. Many so-called Young Adult Fantasy books try too hard to identify with a presumably younger audience and start dwelling endlessly with "the drama of growing up" and constantly pointing out how young and inexperienced the characters are. This book presents a very sincere and straightforward approach to the subject. There are very few mentions of Karl's and Esther's age, and neither of them reflect on their helplessness; still, you never forget that they're both teenagers. The subject is treated with such subtlety and sensibility you would almost expect the book to have been written by someone who is facing those issues right now.
My only problem with this book is that now I'll have to wait until the second volume is out!
(PS: English is not my first language, so please forgive any misspellings or strange expressions I might have used)