It's a series of conspiracy theory books

David Paulides is a former police officer who is now an investigator and writer known primarily for his self-published books dedicated to proving the reality of Bigfoot, and establishing the Missing 411 conspiracy theory. Missing 411 is a series of books and films, which document cases of people who have gone missing in national parks and elsewhere, and assert that these cases are unusual and mysterious, contrary to data analysis which suggests that they are not actually statistically mysterious or even unexpected.[1][2]

The author ^ is a Bigfoot crank and studies have shown these missing people are normal for national parks. But yeah this case is super creepy, especially the kid talking about the cave systems too.