In the introduction, Father Amorth openly states that this book is not intended to demonstrate truths such as the existence of demons, the reality of demonic possession, and the power to expel demons, because those truths have already been revealed and are constantly taught by the Magisterium. Instead, Amorth wrote this book to serve as guidelines for other exorcists because it’s so hard to become learned in this area, largely because there is no longer any master/apprentice type of learning process.
I had two key takeaways from the book, which I’m not sure Fr. Amorth intended:
By seeing accounts of the tangible effects of witchcraft, in a weird way, it strengthens your faith in the sacraments, in that these aren't just empty rituals that are being performed, there is a deep, substantial impact on the participants. By demonstrating and showcasing the spiritual powers of evil, in a strange way it also reaffirms the spiritual powers of good which were handed down through the Apostolic succession of the priesthood through the sacrament of Holy Orders.
But perhaps the most important aspect of the book is that it makes you reflect how your own life is similar to that of those who are possessed. Like them, Satan’s influence also ebbs and flows with you, based on how frequently you receive the sacraments, pray, and make an effort to lead a good Christian life.