Back in the mid-90s, I came across one novel in a series, the title and author of which I absolutely cannot remember right now.
It was a vampire novel series, but the vampires were actually parasitic creatures that took over human bodies. They weren’t humanoid in their native form. They MAY have actually been ETs, but I don’t remember that for sure.
The books themselves centered on a government agent who was hunting these vampires down, or maybe trying to get to the center of a conspiracy between these vampire parasites and the government officials.
My brain keeps trying to say the author was Robert Ludlum, but that’s got to be wrong, and I think I remember noting my confusion even back then.
You don’t mean “The Puppetmasters” by Robert Heinlein, do you? It’s got non-humanoid parasitic creatures that control humans, a government agent trying to stop them, and an author named “Robert” (and it was republished in 1990).