"Were they truly intelligent? By themselves, that is? I don't know and I don't know how we can ever find out..."
While the idea of alien parasites infiltrating humanity is pretty much standard sci-fi now (from Jack Finney to "Stargate SG-1"), Robert Heinlein was pretty early on the concept. And "The Puppet Masters" remains a chilling story to this day -- he wove together some brilliantly vivid writing, some climatic twists, and an intelligent look at how the threat of alien slugs would change our society almost overnight.
Sam (an agent for a top-top-top-secret government organization) accompanies the Old Man and his new partner Mary to a site where a UFO supposedly crashed in rural Iowa. Unfortunately, they soon encounter bizarre gloppy alien creatures that attach themselves to a host's back -- and it turns out that one of them sneaked along with the Old Man's team, back to Washington.
With Iowa completely possessed and the government threatened by alien manipulation, all of humanity suddenly is in danger -- countries start bickering, people become hysterical, and almost everybody is practically naked. As the United States tries to keep the aliens contained, Sam and Mary must find a weakness in the puppet-masters that won't kill the host as well. And the answer may lie long ago in Mary's half-forgotten past...
"The Puppet Masters" is a true classic -- it spawned "Invasion of the Body Snatchers," "The Faculty" and even a "Star Trek" episode. Not only is a chilling look at a quiet alien invasion via "body-snatching" slugs, it's also a pretty intelligent look at the societal changes that might come from alien parasites -- clothes aren't worn, pets become lethal, and an atmosphere of distrust where anyone may become a possessed killer.
The biggest problem with Heinlein's writing is the sexist attitudes towards "females," which is smugly condescending at best. Otherwise he comes up with a pretty solid "future" Earth that is just a little more advanced than we are and a few wars down the road (World War III is mentioned), but not too different in the ways that count (if you can overlook now-anachronistic stuff like a communist Russia).
And Heinlein unrolls a slow-moving sci-fi tale that's heavy on the social/political stuff, some horrific moments (S "All planets are ours") and a rapid romance between Sam and Mary. His style has a delightfully, deceptively casual flair and some snappy dialogue ("Cosmetics?" "Your own ugly face will do"), but he also does a brilliant job with the more atmospheric, intense moments of the book -- such as a blissed-out, hag-ridden Sam drifting around Washington.
Sam makes a good sharp-tongued, quick-witted hero who still has time to feel sorry about killing a poor innocent cat, although Mary is somewhat two-dimensional until the end of the book (when we find out more about her). The Old Man is perhaps the most compelling character: an incredibly smart and ruthless chief of a government agency, who cares deeply about his estranged son but is still willing to put almost everything on the line to save humanity.
Aliens taking over human bodies is something of a cliche now, but "Puppet Masters" is a suitably chilling look at the trope's origins. If you can get past the antiquated attitudes towards women, it's a brilliant little book.