I read the actual Keel book, and really enjoyed it. It is a journey of a man looking for flying saucers who goes down a road in which subtle change, introduced in a slow and hardly noticable manner, leads him to conclude that before a great ill befalls a place some people will experience being taken over by others, not space aliens or monsters, but something else.
The movie, however, introduced about 1/3 of the elements of the book and was a steaming pile of dreck with no element of anything the watcher could believe, and characters we could not care less about.
And then there was Gere and his acting 'ability'...

Wasn't that filmed in Kittanning, PA?