Can you recommend a great horror book please?

There are not as many honestly scary horror stories as you would think. If you want to be truly afraid, the only novel I can say has really really frightened me is "Ghost Story" by Peter Straub. I can also recommend "The Haunting of Hill House" by Shirley Jackson - the greatest haunted house novel ever written according to Stephen King. A lot of the best horror stories are in the form of short stories - and you can usually pick up horror anthologies easily and cheaply.