The three boys are presumed to have become trapped in a cave they were exploring after it collapsed on them, but a search turned up nothing. However, as the article notes, the search might not have been as thorough as it should have been. I suppose there’s always the possibility that they were abducted or something, but I think that’s quite remote.
There’s a book about the case, but from the reviews I don’t know if it’s worth the price tag of almost $16. And my library doesn’t have it — in fact, according to Worldcat, no libraries have it. It seems to me that if an author wants to promote their book they should send some free copies to a bunch of libraries, at least locally. I can’t tell you how many wonderful books I’ve found just randomly browsing shelves.