I have just read this book. My Spanish is not great (say about AS level equivalent), but I found that I could follow the story even without understanding every single word. The build up is superb and the ending quite emotional. There are parts towards the end where (without giving anything away) the action can get a bit tedious - but it was essentially written for teenagers and I imagine they would love it - it would certainly make a very good vehicle for a Guillermo del Toro film.
There is a good and varied use of vocabulary which has certainly helped to improve my Spanish. On the strength of this, I have just bought his "El Palacio del Medianoche".
I too am amazed that a previous reviewer can give a bad rating on the basis of it not being written in English when the item description clearly states the language to be Spanish