This is one of those "horror" novels in which a lot of people have a lot of sex because demons. Or maybe just one demon who somehow possesses multiple people at the same time. Seriously, if you take out all the descriptions of people's naked body parts and their thoughts about assorted sex acts and all the sex they're having, like 3/4 of the book would be gone. So if you're into that, this book is for you. Personally, I prefer more horror in my horror.
The "plot":
This youngish couple moves into an apparently shitty house in the worst part of town because the husband wants to even though the wife doesn't. Incidentally, she's the only one bringing in any income, 'cuz hubby's going to law school, which also means she's alone most of the time in said house in said worst part of town. The day they move in they throw a party, at which there's an orgy that's demon-fueled, and then after nobody except Wife remembers any of it, a guest performs a demon-summoning ritual, but doesn't tell anyone that's actually what she's doing. But also, the demon turns out to have been around since WWII, so the ritual actually has nothing to do with anything.
Wife (who is practically a nun) gets the most possessed by this Nazi-summoned demon, and this is where maybe 75-80% of the over-described sex stuff comes from. Like, basically all she does for 200 to 300 pages is be naked and masturbate and force people into having sex with her.
Eventually (like when there's maybe 80 pages left) a demonologist and Husband fight the demon, and then the book is done.
So basically there's a bunch of porn, some people (mostly randos) and a dog die, and finally God decides to step in after 40-some years. The end.