I know I was not prepared for this. I love horror movies and books, have seen the Exorcist, the Omen, Rosemary's Baby, The Exorcist III, etc. I have read both the Exorcist and it's sequel, Legion. And I have even seen the Warrens' seminar four times. I sat down with this book expecting a good scare and some good entertainment. What I got was something else altogether.
To start, The Exorcist is a cakewalk compared to this. It puts any Stephen King or H. P. Lovecraft story to shame. It's simply the most frightening book I have ever read. This is partly due to how convincing it is and partly do to how it portrays real horror. It describes the intelligence and strategy of the demonic spirit, and the insanity, terror, and consequences they're capable of. The Exorcist seems like a very mild case of exorcism after reading this.
But more importantly, this book makes you think. It wasn't written to scare or even to shock. It was written to inform. I didn't come into this a believer, either in the supernatural or religion. I was a curious enthusiast who found the Warrens to be fascinating, even if they are just conartists. (Maybe they are, maybe they aren't. I don't care.) But since I've finished it I've had to completely reevaluate my stance on religion. I think it gives a deeper understanding of true evil and a very real God. This book didn't make me run out and find Jesus, but it did make me rethink religion. One very important reason for this is that the book, even though the Warrens are Catholics, doesn't cater to one faith. Instead it paints a picture of religion that includes all faiths. It basically backs up the belief that all major religions worship the same God, just in different ways.
The case for the existence of demonic spirits is so carefully thought out, so tactfully delivered that one can't help but wonder. I couldn't help starting to believe. This is a very different type of book than The Exorcist or Rosemary's Baby. For one I found reading it at night unbearable.
The book goes on to make the bizarre and ridiculous stories like Amityville seem so much more possible. After reading this the so-called "evidence" that these stories are hoaxes will seem laughable. It will be clear that those people just don't understand.
Many reviews here will tell you that strange things happened to the reader while they were reading the book. This is ridiculous and untrue. For one, this book is doing just what the Devil doesn't want, exposing his truth. Reading it is quite safe and will make you less likely to experience something like this. Knowledge is key. The greatest trick the Devil ever played was convincing the world that he doesn't exist.
So I urge you to read this. You won't forget it. You won't regret it.