I think where some people are nonplussed by this novel is that they come to it expecting a classic haunted house story, and it is that of course, but it's also a subtle look at the steady disintegration of a young woman's mind. I first read it myself several years ago and didn't like it, I was expecting it to be more like the film. I came back to it again recently and have been completely swept away at Ms Jackson's skill as an author.
There are several references to real life haunted house case histories, as in Borley Rectory and Ballechin House, and clearly Ms Jackson was influenced by these, in that she wanted to write a story in which a small party of ghost-hunters hole up in a haunted house to see what will happen. Where this differs from all that though is that Hill House itself appears to be the evil entity, not the ghosts it may contain. She constantly refers to it as a mad place, with a mad appearance. Eleanor Vance, one of the party, is a young woman who has spent many years nursing her invalid mother and missing out on life. She has become deeply introverted and neurotic. (I felt there were some comparisons with Catherine Deneuve's character in "Repulsion"). She wants desperately to belong somewhere, but when the group she finds herself in start to act like a family, i.e joshing each other, teasing etc, she can't cope with it. Some of the early scenes, when the characters are getting to know each other, were reminiscent of "Big Brother". You felt Eleanor needed a Diary Room to retreat to and voice her concerns at! The ghost-hunters do become a family, with Dr Montague, the genial old academic, as the father figure. Hill House itself at times becomes to resemble a comfy sanitorium, with the younger people reduced to a childlike state, spending their days eating and exploring the house and grounds.
The spook factors may be too subtle for some people's tastes. This is no Richard Laymon-style horror with machete-wielding psychopaths leaping out of the woodwork. The most frightening things that happen are the hammerings on the walls. What makes this novel great is the way it shows a small bunch of people acting in an unusual set-up, and of course, the final chilling realisation that Hill House wants Eleanor as it's captive. Stephen King in his book "Danse Macabre" said that he found this fact too horrifying for words. Highly recommended.