This is the sequel to The Exorcist. It's about demonic possession but not about exorcism. The frontline of defense against the demonic serial killer is a wisecracking Jewish cop who spends most of the novel asking himself religious questions (mostly on theodicy, the problem of the existence of evil in a universe ruled by an omnipotent god) and annoying everyone around him with impenetrable remarks. Imagine Columbo if he just couldn't stop yakking and fancied himself a theologion but didn't understand evolution.
The religious rambling is tiresome, the demonic stuff is unscary, and the plot is shoddy and full of holes. There was one mildly interesting character, a grief-wracked neurologist with a terminal illness who picks up voices from the beyond on his tape recorder, but he wasn't well integrated into the plot. There are a couple of dream sequences that I liked. The wrap-up comes when the detective polishes up his grand religious theory of everything, which doesn't exactly tie up all the loose ends.
If you want to read something scary about exorcism and demonic possession, I recommend Hostage to the Devil by Malachi Martin. It purports to be true but is almost certainly 99% fiction. Even so, it's thought-provoking and genuinely terrifying. Unlike this.