I found this book intriguing but at the same time came away from the book a bit dissatisfied.
I'll discuss some of the negatives first. I don't think that this book is appropriate for most children at the middle school level. There is nothing inappropriate or gratuitous in the book but Cooney describes with a coolness almost the dispatching of various prisoners on the march. Of course I realize that such actions actually occurred on that march/capture. I don't want students to be unaware of what happened but I think that some aspects of history should be discussed at later ages.
Now for the positives. Cooney deals with a lot of serious issues with admirable ease. In Mercy Carter she creates a believable and likeable character that struggles with very real issues.
The book is an excellent book, a pretty good read.