Today I finished A Plague of Demons, a nebula award nominated novel from 1965. I've read plenty of Keith Laumer, particularly the Retief series (for example Retief's War, also on Free SF Online). He has a solid grasp of straight-up adventure, rewarding for a teenage boy to read.
A Plague of Demons revolves around the premise that earth has been secretly invaded by a group of nasty superior beings that steal soldiers for their wars, among other things. Not a lot of development, other than that they are mean. Our protagonist is outfitted with experimental augmentations to battle them. He is assisted by a secret society dedicated to the preservation of civilization. All these were prominent tropes in 60's SF, including campy TV shows (The Man from U.N.C.L.E., Get Smart, etc.). The novel is decent up through the part where the protagonist is captured and put in a robot himself--it gets confusing there, but drives on relentlessly. Just OK as a read. 2 stars