One of the really interesting things is to go to Catholic Answers radio section of their site.
If you go into their archives, you can go to an interview with Fr. James Lebar(sp?) an Exorcist for the New York archdiocese at the time.
A caller asked him if he knew or read "Hostage to the Devil" and whether it had any kind of truth in it.
Lebar responded, "Yes to both. I know Fr. Martin and he needs our prayers as he is ill currently and is elderly. But the book Hostage to the Devil is very accurate and pretty much tells it like it is."
Then Lebar goes on to relate stories of Exorcism that go way beyond any of the physica manifestations that Fr. Martin described. One Lebar referenced was when a bishop was in a hospital to give last rites to a dying priest. As he went past a room, a person was climbing on the ceiling and cursing the bishop.
The bishop ignored the possessed and went on to give the priest last rites. When he came back up to the floor of the possessed. He started the rite of exorcism. Within minutes the demone howled that he's failed and that the priest had died at that moment and been taken to Heaven.
I can't recall where I read this story. A priest went to a locked down psychiatric hospital to bring Holy Communion to a patient. The Eucharist was concealed in his clothing as he walked down the hallway, but as he passed several of the rooms the patients inside became out of control with agitation, screaming at the priest as he walked by. It was a strong case for demonic possession in psychiatric cases.