I do not remember how I first found out about the books, but I do remember listening to the youtube recording of the author David Paulides on Coast to Coast AM talking about the book.
There are now 3 in the series; Western US, Eastern US, and now North America and Beyond. It's written by a former police detective and covers areas across the country, a lot in national parks, where there are clusters of people, mainly young kids, who go missing, many of whom are never found again. Many times the person when missing out of a group of people, or stepped behind some bushes and never came back out. It also covers the apparent cover up by the national park service of the disappearances as they refuse to investigate and claim they do not keep records of people who go missing in the parks. It's very interesting and something that anyone who spends time out in the woods should read as you may learn of dangers that most do not know about. It's written in a format listed by the area of the country and then starts from the oldest to the newest cases. It's worth the read. I am 1/3 through the second book, just saw the 3rd book a few minutes ago.