Mr Card, tells us about the mistake that many writers fall into. That of beginning a book with one story and ending it with another.
“[T]he beginning must make the audience ask questions that are answered by the story’s ending, so that when they reach that ending, they recognize that the story is over.
The beginning of a story creates tension in the audience, makes them feel a need. The ending of that story comes when that tension is eased, when that need is satisfied. So in determining your structure, it is essential for you to make sure your beginning creates the need that your ending will satisfy; or that your ending satisfies the need that your beginning created!
—Orson Scott Card, How To Write Science Fiction and Fantasy