The supernatural element is disturbing and creepy, and it only gradually dawns on the reader that there might be another explanation other than the rather obvious haunting initially suggested. From the reviews, some people seem to think the twists and ambiguities in the novel end here, but I disagree. There's ample evidence of multiple theories - that there is a specific spirit haunting the house; that someone living is psychically responsible; that someone living is responsible in a rather more mundane way; or that the house itself, like Hill House before it, is preying on each victim's weaknesses. There doesn't appear to me to be any definitive explanation (indeed several of the above could overlap) and foe me the book displays the same kind of eerie ambiguity as The Turn Of The Screw.
What frightens us most is what we don't know.