Ideally, these two words describe the ending of your book. The question is, how do you achieve it? Not every story can be as clever as “Gift of the Magi,” O’Henry’s most famous story.
The key, I think, is the same one used by stage magicians: misdirection. The problem is, if the reader thinks about the story elements in the right way, the answer will be clear. The solution is to offer them a false explanation for events. The first novel in the Mistborn series does this when it tells you (spoiler alert) that the Lord Ruler wears metal rings to show his bravado (wearing metal is dangerous for allomancers, the main type of wizard in the book). Some of the other nobility copy this fashion statement, reinforcing its seeming truth in the mind of the reader. None suspects that the Lord Ruler is also a ferrochemist.
Another way misdirection can be used is to call the readers attention away from the critical element, not bringing it into play until it’s too late. In Star Wars, we don’t expect Han Solo to save the day in the end. He’s not a good guy–after all he shot Guido first. All through the movie he’s consistently interested in money, and before the climactic battle he deserts the heroes. It is his change of heart that provides the unexpected element in the inevitable outcome of the attack on the death star.
What other examples of misdirection to provide the unexpected ending can you find? Does thinking about the use of misdirection to provide the unexpected element of your ending help you to plan the ending better?