The Puppet Masters - Robert Heinlein (1951)

Classic SF Noir displaying America's paranoia in what has always been for me Heinlein's best novel. It exemplifies all that is good about mainstream SF of the Nineteen Fifties and suffers only from minor political incorrectness in terms of male and female stereotyping, and the rather irritating remark made about gay men by the US President; 'There have always been such unfortunates.'
But then, Heinlein is rather on the right wing of the SF stalwarts of the time, and this is a peculiarly masculine novel. We are told in the first few pages that the entrance to the secret headquarters of a government department so secret it doesn't even have a name is situated in the men's washroom on Macarthur Station. The women (for thankfully there is at least one female agent) no doubt use the other entrance situated in a shop called 'Rare Stamps and Coins.'
Our hero, Sam Nivens, is a square-jawed All American type who would willingly die to preserve the liberty of America and whose laconic monologue tells the tale of the invasion of the Puppet Masters.
A rather decent TV movie of the book was made with Donald Sutherland in the role of 'The Old Man', the hard-nosed boss of the Department. Although surprisingly faithful to the text of the novel it suffered in that it was set in the present day. It should really have been made in black and white and visualised as a Nineteen Fifties view of America in 2007.
Heinlein's aliens, a perfect metaphor for what America believed typified the evils of Communism, are a kind of gestalt entity; grey slugs which attach themselves to the backs of humans and take over the mind and body of their hosts. They are sexless, appear to have no individual personalities and exchange information by some form of physical transference when in direct contact with each other.
Just as in 'The Body Snatchers' (Jack Finney, 1955) the aliens 'infect' humans by stealth, reinforcing the idea of communism as a plague, contagious, insidious and more than anything else, invisible.
The hosts are literally enslaved by their masters ('Master' actually being a term which Sam uses to describe them). Heinlein takes these threats of loss of individuality, the natural fear of disease and the rather disturbing concept of slavery (which is as alive and well today in the guilty American consciousness as it was in Nineteen Fifty One) and winds them all together into a chilling tale of what is essentially a war of ideologies.
I imagine a writer of today would not make the story so one-sided. In a sense this novel says a lot about Heinlein. The book might well have been stronger if there had at least been some benefit, or purpose to the aliens' invasion. As it is the aliens do not compel their hosts to wash or eat properly, and so are destroying the hand that feeds them, as in when it is discovered that the bubonic plague has returned to Communist Russia.
No system is truly evil. If Heinlein consciously meant these aliens to be metaphors for Communism then he should have made them less unknowable. The suggestion is that one shouldn't even try to understand Communism. To attempt to know Communism is to be infected by it. The menace cannot be lived with. It has to be eradicated from our minds.
Of course, it's difficult to understand, in a post USSR world, what level of paranoia existed in America at the time.
Certainly, whether consciously or not, a large number of SF films and novels of the time featured ordinary people being 'possessed' by aliens, often taking over an entire community, abandoning American culture and values and replacing it with something else.
Sam - who eats steak 'just warmed through' - needs to prove to a sceptical President that the aliens exist. His plan fails and when a live slug is eventually captured, Sam is 'possessed' and for a while we see the world of the 'hag ridden' through his submissive eyes. It is this experience which elevates Sam from a mere two-dimensional hero into something greater. A stereotype he may be, but in Nineteen Fifty One it is interesting to see an SF hero with fears, emotions and failings, and who even cries on occasions.
Of course, with the help of his partner - an efficient female agent with a taste for weaponry - the world is saved and Sam spearheads a military operation aimed at saving the elf-like denizens of Titan from the curse of the Puppet Masters. This suggests, one presumes, that even back in Nineteen Fifty One Americans felt they had a duty to right wrongs beyond their own borders. The aliens themselves are beautifully thought out. An immortal gestalt entity which reproduces additional units of itself by binary fission and may which hold memories dating back to the dawn of its sapience.
At the end of the novel they remain enigmatic, and the question, raised in the opening paragraph of the book as to whether they are intelligent in any way we understand, is never answered.