The Puppet Masters (original) Robert A. Heinlein

First came the news that a flying saucer had landed in Iowa. Then came the announcement that the whole thing was a hoax. End of story. Case closed.

Except that two agents of the most secret intelligence agency in the U.S. government were on the scene and disappeared without reporting back. Then four more follow up agents also disappeared. So the head of the agency and his two top agents went in and managed to get out with their discovery: an invasion is underway by slug-like aliens who can touch a human and completely control his or her mind. What the humans know, they know. What the slugs want, no matter what, the human will do. And most of Iowa is already under their control.