This is book 4 of a series about Henning Juul, an investigative reporter. According to reviews, it works okay as a stand alone and plenty of backstory is provided. One reviewer sums up previous events here,
https://www.goodreads.com/review/show/1923867288?utm_campaign=reviews&utm_medium=widget.
She then goes on to say: Mr. Enger, if I can have a word with you regarding the last page
.WHAT?! All I can say is book #5 better be coming out PDQ.
Another reviewer says, "That last page!! *gasp*. I'm sure if you've read any other reviews surrounding this book you'll realize there is a huge WOW moment in the last few sentences. The hype for once is worthy; the ending shocked me to the point I actually gasped out loud."
I was hoping to find this book through my librarys digital collection so I could either read or listen to that last page, but alas, nobodys got it. So, I just have to say that you are not alone in your confusion. That and you have convinced at least one person to go out and buy the book now.