No one has any idea why Amber has returned to the living but the fact that she has crystalizes how much damage her death wrought in all their lives. Her family becomes convinced that they need to restart their lives, even if it means running away from their current ones. They certainly can't stay. After all, what would happen if the rest of the world discovered that Amber didn't really die seven years ago? But there are lots of forces coming together and a complicated web of relationships and interactions that may well make Amber's return moot.
The novel's structure, with current state chapters told from Amber's point of view alternating with flashbacks from a variety of others creates a very dense story that relies on a combination of coincidences to make it come together. Nothing really makes sense until the end. Yet it works surprisingly well. The novel's message -- that the "life" of a person after their death is largely dependent upon how those whose survive them behave -- is told in a variety of fascinating ways ranging from parents, children, and even pet owners. Ultimately, it is a very touching story that lingers with you.
