This would make a very scary movie. As a book - it was entertaining - but it did not make my imagination go wild at the thought of the spirits inhabiting Hell House. 
The book is a spin off on the "I'll pay you $100,000 to spend the night in this totally creepy Haunted House." Instead - in 1970 a wealthy man hires three people, two paranormalists and one parapsychologist, to spend one week at Hell House with the goal of confirming, or debunking, the presence of other spirits in the house - and - if they exist - clearing the house of their presence. 
Florence Tanner, a spiritualist; Benjamin Franklin Fischer, the lone survivor of a 1940 visit to Hell House - where Fischer felt the presence of spirits; Lionel Barrett is the scientist, afflicted by Polio and accompanied on the trip by his wife Edith.
While they are there - Florence becomes a portal for other spirits that try to send messages to the group - including Daniel Bellasco - the son of the owner of the house who apparently died an angry young man. Each member of the quartet has their interaction with the spirit that inhabits Hell House - to varying results. Two die, two survive.