I've been away from the Ember Wars for a while now, as it wandered off from the core story and characters several books ago, and that didn't really hold my attention.
The events of this book have been mentioned in the main sequence of EW books and we had a pretty good idea of what happened, but no details.
Probably a good thing, as this can't have been an easy book to write. It spoils nothing to tell you that everyone in this book dies. EVERYONE. While it wasn't the most fun thing I've read, it was well done and the pacing was very good.
I'm split on whether I'd recommend this to someone as 'book zero' in the Ember Wars, but it could work. That may have been the author's plan, to draw eyeballs back to the original series, but what do I know?