This is the fourth installment of David's work on missing people in national parks. First thing to be said, this is NOT available on Amazon. It is available from David's site. Much like the previous books, this is a collection of cases that fit certain criteria. There are details in the cases that David focuses on, and he lays everything out pretty clearly. In some cases there is a lot of detail, whereas in others, the information is sparse. I am sure, in the end, some of these cases will be found to have prosaic explanations. Some of them, however, are very unusual. In this book, David has found the first cases that involve water disappearances that also fit some of the criteria. These cases are strange. If you have not read the previous books, you can still start on this one if you want. Everything is well explained, but reading all the books does give you a better idea of how strange these disappearances are. These are not cases of animal attacks, or serial killers, or serial kidnappers. There is something here we are missing, that we are not aware of, that is happening to these people. You can't really get an understanding of just how odd it all is, without reading the books. The cases are separated by state, and in the back are charts with various key points outlined as David tries to flesh out the patterns. This book is written pretty plainly, it's not told in story form, just facts from various sources, sometimes with comments from David afterwards. This really is a bewildering mystery, and it's a hard to put down book. It definitely makes you think twice about wandering national parks alone.