I think every book is, in essence, a mystery novel because the way to keep a reader interested is to throw out hints of things to come, things the reader wants to know the answer to. If nothing else, what in the world is going to happen next?
For instance, in "Little Red Ridinghood," we wouldn't have a story if the wolf hadn't approached Lil' Red early in the game. The mystery is what the wolf will do next. Or, even more basic, why in the world did the author introduce a hungry wolf, and how will the leering lupine satisfy his horrible hunger in a way that involves our hooded heroine?
Ooops, --do I feel an erotica coming on?