Eric Frank Russell's "Three To Conquer" aka "Call Him Dead" (novel, alien invasion): Invasion by intelligent microbial parasites of Venus
This is one sitting reading material. Fast paced, unputdownable, action story. But based on much written about ideas: ghosts & alien forms "possessing" a victim, & inhuman beings masquerading as humans quietly on their way to replacing humanity.
Story summary.US government has sent a secret manned mission to Venus with 3 astronauts: William Gould, Cory McDonald, & Earl James Langley. With two expected dates of their return, depending on whether they find the locate hostile & need to immediately return, or if they get some time to explore. First date is past but not the second - so officialdom is happy exploration is happening.
Note: I'm puzzled by stories even from 1950s assuming manned space travel is easier than space communications!
Actually, officials are wrong. The ship has returned. Only, its three astronauts are now possessed by a parasitic intelligent life form of Venus that by itself has the form of microbes inhabiting the nervous tissue of its host. It's happy in its human hosts, & has quietly set about multiplying itself by infecting more people.
Then we have the hero - Wade Harper. Only known human telepath, & a far better detective that ordinary cops & FBI men. He will stumble upon the secret of aliens, & persuade the powers-that-be in Washington to mount the concerted government effort to rid the world of menace. And succeed, after much drama.
Fact sheet.First published: Astounding, August/September/October 1955 as "Call Him Dead".
Among the stories from Astounding/Analog issues edited by John W Campbell, Jr.
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Related: Stories of Eric Frank Russell (annotated list).