A stray mention in the late William H. Patterson's bio of Robert A. Heinlein caused me to put this oldie in my to-be-read pile. It's the alternative version. It's longer and has more risqué sexual references than the bowdlerized 1951 version. Which I probably last read over fifty years ago—I was a Heinlein obsessive in my youth.
It's a tale of alien invasion, as seen from the viewpoint of "Sam", a crack secret agent working for a shadowy federal department charged with putting out troublesome fires around the world. But their latest call to action is in rural Iowa, where a flying saucer is alleged to have landed. Previous agents sent in have gone silent, so the A-Team, containing the "Old Man" (Sam's boss) and "Mary" (va-va-voom, Sam's about-to-be love interest) fly in. They get to Iowa to find an obvious hoax: a "UFO" constructed from cheap sheet metal and aluminum-sprayed plastic.
But they also find some pretty disgusting aliens, gelatinous parasites that attach to host nervous systems and take over the host's actions. Ish! They barely escape with their lives.
The rest of the book deals with the country's efforts to deal with the invasion, a remarkably tricky task. It doesn't help when Sam is captured by the aliens, and … well, that's enough to say without further spoilers.
The version I got from Amazon (link o'er there) has an introduction by William H. Patterson, Jr. and a long afterword from Sarah A. Hoyt. Both note the strong undercurrent of individualism and freedom running through the book. Ms. Hoyt's words are especially personal and moving. I've always thought that Heinlein exerted a major push to get me where my views are today, and Ms. Hoyt obviously feels the same in her case.