Why I have this book: I bought the series for my son for Christmas last year, and he finished it this past week. He was so upset at the ending and so emotionally invested in it that I told him I'd read it, too. This is the first book of the series.
I gave it three stars on Goodreads. I'm not really into fantasy books, and this one is definitely fantasy. I liked it well enough to keep going in the series.
There are some deaths including animals (who speak and act with human characteristics) and "underland humans" (if that's what they are called), and those deaths are generally by war-type violence. I don't recall any swearing. The book is written for 9-12 year olds, so I think it's as clean as can be in a fantasy war story.