You might enjoy reading Danse Macabre by Stephen King. It was written some time ago, but much of what he has to say still has relevance.
There is also an audio version. If your local library uses Overdrive it's a simple download.
Well said.I think what makes a horror novel great is the same thing that makes any other novel great. As oxygentent said, the characters have to be good... even the bad ones. Without characters that the reader cares about, the writer won't be able to generate fear in the reader that a character might die.
I agree that the plot needs to be possible. That is not to say that the plot needs to actually be able to happen. Instead, I mean that it needs to fit within the confines of the world created by the author. H.P. Lovecraft's monsters are so far fetched that they're almost cartoonish. Yet, they fit within the world he created and they are utterly terrifying in it.
In the end though, with these two things combined to create a believable world filled with characters we care about, the true horror comes in the human story behind the monsters. It's Victor Frankenstein digging up bodies to fulfill his obsession; it's Abraham van Helsing helping put his friend's beloved Lucy to final rest; it's Rick Grimes waking up in the hospital with the world gone to shit and not knowing where his wife and son are. These are the things that draw a reader in and keep them on the edge of their seats while reading and hiding under the covers when they're not.
I think what makes a horror novel great is the same thing that makes any other novel great. As oxygentent said, the characters have to be good... even the bad ones. Without characters that the reader cares about, the writer won't be able to generate fear in the reader that a character might die.
To me, horror has always been visceral, not visual. That's why blood, guts, vile creatures and bleeding body parts flying around the room are not things I find horrific. In other words, it's not watching the monster chew your character's head off that's horrific. Your character's impending and growing fear of that happening is the real horror.
It's a pretty subjective question. Not all things are objectively horrific, because different people will find different things horrific. I have friends who didn't find Mama by guillermo del toro scary.. but that movie scared the living shit out of me. Horror, in the end, is an emotion .. not a genre. Write something that would scare you, and you've got about 6 billion chances of finding someone else whom it'll also scare.