The Sentinel is also a novel by Jeffrey Konvitz, published in 1974. The story is in a similar vein to Rosemary's Baby and The Exorcist, dealing with demonic possession and supernatural forces.
From the jacket: "A beautiful young model. The old brownstone apartment she simply had to have. The grotesque blind priest who watched down on her day and night from an upper-storey window. The pair of perverted creatures who wanted her to join their circle. The mad little old man who gave her tea and sympathy. The cool, calculating, supremely rational lover who first mocked her fears. And the secret you will never be able to forget, even if you try..."
I found a rather dog-eared copy of this book in a second hand shop, obviously an original copy from around the time it was first published. Books about demonic possession look dated today, they were a fad of the 60's and 70's, but nevertheless I have a soft spot for the genre having enjoyed The Exorcist, Rosemary's Baby, The Amitiville Horror and a number of lesser known titles from the period. The Sentinel is one of the better known examples. Apparently there is also a film.