With a bit more research, I see that Colin Wilson also wrote "Space Vampires" with alternate title "Lifeforce" that was made into a movie. Also noted that Dr. Armstrong in this movie is played by Patrick Stewart. It has the continuing theme of the previous two books. Generally the movie does not get great ratings.
Honestly, I like the movie better than the book. The movie is entertaining; the book is just tedious. Or so I recall.