Moral Event Horizon: Cleaver attempts to asphyxiate Crowley's wife and knocks her out with a blunt object. It isn't fatal, but he seems to want it to be. This trope is then lampshaded in conversation when he calls his therapist.
The entire first chapter explains exactly how a body is embalmed in a mortuary. And it has no relevance to the plot. It just serves as one long Establishing Character Moment for John.
This whole damn book. Just the style of writing and how the reader gets so engrossed in John's warped mind is almost enough to give someone PTSD.
Spiritual Successor: The Film of the Book in particular to Donnie Darko. They're both relatively indie supernatural films dealing with a mentally ill protagonist struggling with right and wrong. Both are backed by tumultuous family situations and focus in particular on mother/son relationships. Like Donnie, John lashes out at his mother despite her best efforts, but ultimately reconciles with her. The dark tone balanced with emotion and humor has also drawn comparisons.