The premise of the book is that an ordinary guy, with some unordinary emotional baggage, wakes up one day to find he has devil's horns on his head. These horns give him the power to get people to reveal their deepest secrets and desires. He also has the power to give them permission to act on them. As an earlier reviewer said, some of the earlier instances where Ig was figuring out his new powers could be quite humorous, while others were truly horrifying. I don't think it is a stretch to say that every human being has a very dark side to them that shouldn't see the light of day.
So I would have enjoyed exploring that more. That is, everyday regular people confronting permission to do what they always wanted to do. Instead, the tale goes off onto a revenge thriller where the "devil" has to deal with someone who is truly a monster. Some might enjoy a tale like that, but the first part of the book promised to be something more engaging than the typical good versus evil. I wanted to know more about "the monsters" true motivations, but even under the spell of the horns, we learned nothing about what made them that way.
But I do think this is a worthwhile book to read because it certainly makes you think.