I don't really see what the point of this book is, or how it ever became popular. The mystery is terrible - the only plausible answer is "it was magic", which is no kind of answer at all. The characters are entirely one-dimensional and illustrate nothing more than the author's prejudices. There's not even any kind of plot to speak of - any time it seems like something's starting to happen it moves on to something else.

Also, the author doesn't seem to have been paying attention to her own book. She repeatedly points to the disappearances as being the cause of later events that can actually be traced back to events preceding the titular picnic. Like, are we supposed to believe that the French woman wouldn't have gotten married if those girls hadn't disappeared? She already knew the man she ended up with, and there's nothing to indicate that the events of the book brought them any closer together.

So yeah, I don't get it. There's just nothing to this book. No interesting characters, no plot, and certainly no compelling mystery. What do people see in it?