(Short recap: the MISSING 411 books focus on bizarre disappearances that happened in the wild, while trying to prove some nefarious connection exists between them all. Paulides, the author, is best known for researching the existence of Big Foot. Hilarity and drinking ensues for the reader.)
I just got off the phone with my mother, who owns all the MISSING 411 books. According to her, the author's berry bush theory--left unexplained in the first book, which I recently read--goes something like this:
There is a race of Sasquatch that have been imbued with alien technology, giving them the ability to mess with people's memories, teleport people miles away from their last known location, and turn invisible. However, as they are still wild at heart, so they are naturally drawn to berry bushes as subsistence. They consider humans to be the most dangerous game, thus why they kidnap or kill people in national parks/forests.
...In other words, the plot to Predator + berries. Yes, really. These books are published as nonfiction.