Dr. Montague is looking for evidence of a true haunting, and in Hill House, he appears to have found it. If buildings have psychologies, Hill House is one of complete insanity. The house has a terrible reputation among the town's locals, so much so that they are hostile to anyone asking for directions to it. But along with two other assistants, Eleanor helps the good doctor collect evidence of paranormal activity.
This novel qualifies as a horror novel (in fact, it's considered one of the very best in the genre)—though there's no gore. The horror is all psychological, and Jackson is so skilled as a writer, all she needed to do was paint us a picture of Eleanor's loneliness to show how easily the house could play on it. The others have their moments, too, but it's clear that among them, Eleanor is the easiest target.
No matter how cliché the haunted house trope may be, I've not read anyone who's done it better than Shirley Jackson (nor anyone who does a better job of painting someone who is lost emotionally and psychologically). She's one of those writers who achieved being both a good storyteller and a good writer. Her writing is a study in economy on par with Hemingway's. And I think that's one of the reasons this novel is considered more "respectable" than most others in the horror genre. You really can't fault the writing, even if it's not your style. It's really too bad she didn't publish more before her death. Makes her work all that more a treasure.