Wow, did I enjoy reading this. Gidwitz is, first of all, a fantastic storyteller. The tale is pretty complex, and also fairly horrifying, but it moves at a rapid pace and is constantly enlivened by clever jokes, allusions, and tongue-in-cheek "warnings" about what's to come. These last will appeal to bloody-minded readers of all ages, by which I of course mean everyone.
And yet all the while, Gidwitz seems to be communicating this profound wisdom about childhood, and parents, and families, and anything you can think of that's related. I know extremely little about children's literature and especially little about the Grimm's tales, so I can't say much from that perspective. But as an adult, the poignant meditations on growing up, and on what parents can and can't be, struck me among many, many other things.
In fact, this happened so many times that I wondered whether this book is secretly written primarily for adults. I will not be surprised if I return to this when I have my own children, as a guide to their world and my role in it. Highly recommended.