On Tuesday Rich B Knight provided a guest post about why horror movies are scarier than books, if you haven't read it then check it out here. He made some valid points, movies do have some advantages when telling a horror story. However I believe that books have the advantage.
The first reason is that books are more personal than films, they make a more direct connection. In films everything is presented for you, you can see and hear what is happening. That's not to say that films can't make you think, but reading takes a bit more initial effort. More importantly you have more to process, you build the story that the writer has created. The better the writer is the more is resonates in your mind.
Reading also requires your mind to fill in the gaps. A good example of this is The Shining. The Shining is a rare example of both being an excellent film and an excellent book. The difference between the two is stark, in the film it's Jack Nickelson that is a huge part of what makes the film good. In fact he's almost too good, the character is transformed into Jack. In the book it's more subtle, the character builds, layer on layer, in the film it's already there on Jack's face the moment he walks into the hotel.
Filling in the gaps also brings the reader closer to the story, it connects with their memories and experiences. A good film can use a similar method, suggestion is usually scarier than the usual shock tactics that films use. With a book you become more involved in the process, almost a participant rather than an observer.
A great example of what makes written horror so good is David Haynes's 'The Scream of Angels' (if you haven't read it then I recommend you do, check out my review here).It's set in Paris at the turn of the 20th century, now a film could represent the setting well with suitable visuals and audio and I'd happily watch such a film. What the film couldn't do, but the book does is draw you into the atmosphere. The atmosphere of the setting is more than what you see and hear, it's what you can smell, touch and taste.
David achieves this with his writing, even the style of the writing wraps you in the deranged and depraved world of the time. It's a wider world than is available in a film. A deeper world also makes it scarier one.
Why is it more scary?
Again it comes down to filling in the gaps. You know what scares you most. The writer just needs to suggest the fear and the reader's mind does the rest. You'll paint a picture more vivid in your head that a film can ever show.