It depends. Sometimes a child being called 'it' might be fictional, but in some extreme or unusual situations, it could potentially be based on a real occurrence.
The main theme is survival. The boy in the story endures extreme abuse and has to find ways to survive in a very difficult and hostile home environment.
One thing we can learn is the importance of recognizing and stopping child abuse. Pelzer's story shows how bad the situation can get when abuse goes unnoticed. Also, it teaches us about hope. Even in the darkest of times, there can be a glimmer of hope that can lead one to a better life.