It may seem odd to say there are both good and bad messages in this book, but when I thought about it, that's what I decided was an appropriate analysis. On the good side, the siblings in this book are loyal to each other and look out for each other. They have been raised to put their family first, and they frequently think first about how their actions could impact their parents or each other. On the bad side, it is revealed in the book that after the disappearance of his girlfriend, Celia, Whit experiments with, or even takes up for a time, drinking, and while it is evident that he feels some guilt at this, the author does not deal with any consequences of that action beyond Whit's feelings of guilt, which arguably, are also a product of the loss of Celia. That said, I did not list the underage drinking itself as a concern because, while its consequences were not adequately explored, it was also not glorified in any way.
A lot of parents may find this book to be too violent (I did not), and there is a lot of violence, especially violence perpetrated by adults against children. For me however, it was made clear that those engaging in the violence against the children were Evil, and the violence is portrayed as being acts of Evil. The violence engaged in by the children in response is shown as necessary to save the lives of others, and therefore justified. If you have a problem with the concept of violence being justified in the face of violence, you will not approve of this book.
Overall, the book is not especially good. It's not particularly well written, and is a little too colloquial for my taste. It has just over 300 pages, and over 100 "chapters" which are not divided up in any logical fashion that I could discern, aside from a desire to keep each chapter extremely short (which I would think should be unnecessary with kids in this age group). I read it in a couple of hours in one night. My daughter (age 11) has not yet read it, so I cannot relay any comments she may have had about it.