How does the author use foreshadowing in Blue Is for Nightmares?

In Chapter Five, things start getting suspicious when Stacey receives a strange email from Chad based on nursery rhymes about the house that Jack built. She is surprised that he would go into so much trouble to do this. Such emails don't seem to be something that Chad would engage in, and she feels slightly disappointed that she doesn't receive anymore emails from him like before simply saying goodnight. These events foreshadow what is to happen in the future.