Well knock me over with a feather. Commenter David likes this book best of the three by a long margin. He suggests it would work well as a movie.
On reflection, I think he is quite right on that. Many of the book's difficulties disappear in film's medium. There are conventions from the horror and SF movie genres that would make the spiritual and demonic nature of the N.I.C.E much more plausible and efficiently captured. Some scenes that are a bit tedious to read because of the necessity of describing a great deal - such as Jane Studdock's visions or the confusion of tongues scene at the end, might be remarkably easy to show. Feverstone's character, or the Fairy's enjoyment of torturing a certain type, can be shown more easily than described. The bloody attack by the animals, the floor running with blood - okay, that might be tougher to do without good special-effects people. Jane's ambivalence about marriage/career/personality/obedience, and Mark's dawning recognition of what being a husband actually means would have to be put quite differently. The accidents of mid-20th C cultural choices are both too near and too far from our own to resonate well. But as these same questions remain a popular topic now, I'm sure there's a way to accomplish it.
As a movie, I think it works. A rather intense and frightening movie as well. I certainly wouldn't bring children to see it.