This is the first book of the series that really disturbed me, though. There have been a few somewhat gory scenes throughout the series, but never like this: There was a scene where Valkyrie voluntarily died, and spent I think two chapters in a horrifying kind of waking death. She was strapped down to a surgery table and cut apart, had her rib-cage clipped and pulled open like the door of a cabinet, and her heart removed. She was awake the entire time; and eventually she frees herself from the restraints, throws her heart in a bag, apparently closes up her opened chest and wanders the evil doctor's facility.
This scene is described with quite some detail. It's not the only gory scene in the book, but it's definitely the worst. Because of that scene alone, I would definitely no longer classify these as children's books. Teen books, sure, although it's rare I find anything that gruesome even among those shelves. But not children's.
There were also a couple of minor plot-holes that irritated me, and I'm sorry to see a couple of the characters meet the end of the book the way they did like Kenspeckle, but overall, I still loved the book, and I'm looking forward to the next in the series.