"To Serve Man" by Damon Knight

First appeared in Galaxy. Reprinted in various major genre retrospectives by Everett F. Bleiler, T. E. Dikty, Joe W. Haldeman, Mike Ashley, Frederik Pohl, Martin H. Greenberg, Joseph D. Olander, Isaac Asimov, Richard Matheson & Charles G. Waugh, Bruce Coville, Robert Weinberg, Stefan R. Dziemianowicz, Dennis Pepper.  Won a Retro Hugo.  A classic story on the Twilight Zone.

The Kanamit aliens arrive, promising to end the world's troubles--begin endless cheap power, end hunger, end war (explosives will no longer go off).  They want to help.  One representative from Russia, Grigori, is dubious but his hard questions fall on deaf ears once the alien passes a lie-detector test built for humans.  They have a book called How to Serve Man, which appears to validate their good intent.

[Spoiler] It is too easy to spoil this tale, so I'll try to make it harder to spoil with extra words....  [Spoiler] It turns out the Kanamit book is a cookbook.

Clearly the joy of this tale is the surprise--one of extreme audacity, especially leading humans down the opposite path of intention.  However, the tale has some subtlety often overlooked.  The aliens didn't actually lie (well, they did and they didn't).  They created a trail that allowed humans to be misled.  As Grigori puts it:

"They told the truth, though, as far as that went." 

In fact, some of the humans appear complicit in the Kanamit chicanery by not allowing debate.  Knight waits until the last third of the tale to reveal the book title, in order to make sure we were thinking along his terms so that we could be duped like the Earthlings in his tale.  In the tale's last line he reveals the secondary meaning, which sticks with readers years after they've read it.

What did Knight expect to accomplish by making the aliens hideous pigs but bearing gifts?  Probably it makes you feel guilty that you felt suspicious about their motives since they only appear to want to help.  The theme likely comes from Grigori who distrusted the Kanamit all along:

"[T]here is no such thing as a completely disinterested altruism.  In one way or another, they have something to gain"

The Twilight Zone episode changes it by not making the aliens hideous pigs and by setting an initial mood of retrospective anger and anxiety.  This may ruin the tale for some, but not others.  However, Rod Serling also made it personal, had it directly impact the narrator, who is about to be made into dinner, which may have made it zing for viewers.  Also, Serling delivers a great double entendre, "They want to make Earth a garden of Eden."