“The White Mountains,” a young-adult novel first published fifty years ago, is set in the future, at a time when the Earth has been rendered entirely rural and has been taken over by aliens. Towering, inscrutable machines called Tripods stride about like giant Humvees atop metal stilts. Humans attend “capping” ceremonies around the time of their fifteenth birthdays, in which long mechanical tentacles swoop down from the Tripods to yank eager teens skyward, one by one, into their interiors. When the adolescents are returned to the outside, they have mind-control devices woven into their scalps, robbing them of free will—depriving them, more specifically, of any desire to fight the creatures that manipulate the machines. Then a young hero named Will, through a great deal of luck combined with an innate skepticism, figures out what’s coming and flees before being capped. He has adventures on the high seas and amid once great but now abandoned cities, finally finding his way to a small group of free humans working to launch a resistance. “I could not stay, any more than a sheep could walk through a slaughterhouse door once it knew what lay beyond,” he tells us. “And I knew that I would rather die than wear a Cap.”

“The White Mountains” is the first volume in the Tripods Trilogy, by the British author John Christopher, who died in 2012. Christopher’s real name was Sam Youd, and he wrote under several other pseudonyms as well—Hilary Ford, Peter Graaf, Stanley Winchester—ultimately publishing more than fifty novels in all. His first book, from 1952 and long out of print, was a collection of stories, “The Twenty-Second Century.” Roughly half of the stories star a character named Max Larkin, a sort of cross between a corporate scientist and James Bond. The rest are sci-fi tales that hold their own next to Ray Bradbury’s contemporaneous “The Martian Chronicles.”

After the Tripods Trilogy, Christopher turned to young-adult fiction for good. The second volume trilogy, “The City of Gold and Lead,” also published in 1967, unfolds in an enormous alien fortress where slimy, green-skinned, three-footed Masters live outside their Tripods and are served by masses of capped human slaves who, under intense humidity and heavy gravity, can move about only at a crawl. Eventually, these humans either collapse dead of overwork or kill themselves in a final submission to the Masters that is called “Happy Release.” Will arrives at the fortress with the task of becoming a slave himself, in order to learn as much as possible about the Masters so that he can help destroy them. And he’s lucky again, to a point: while many Masters enjoy beating their slaves, Will’s prefers simply to talk. The creature likes to get high on “gas bubbles” and stare beseechingly into Will’s eyes while stroking his face with a slippery tentacle. Will becomes, as one chapter puts it, “My Master’s Cat.”

The Tripods Trilogy was popular right away—the final installment, “The Pool of Fire,” arrived in 1968—particularly in the United Kingdom, where, in the nineteen-eighties, the books were transformed into a BBC series. The show has its own cult following: “The Tripods” Ultimate Fan Meeting Event, for English and German fans of the show, will be held in West Sussex this fall. After the show aired, Christopher published a prequel to the series, “When the Tripods Came,” set hundreds of years before the action of the trilogy. In it, the alien invaders appear at first to be quite easily defeated—they are even joked about on a popular new TV series, “The Trippy Show.” But people grow addicted to the program, watching it incessantly on their VCRs; then, when the aliens launch a second invasion, these same people leave home to serve the Tripods, mysteriously brainwashed by what they’ve seen. Christopher later explained, in an interview, that he decided to write a prequel after a well-known British sci-fi writer mocked the improbabilities of a Tripod takeover on a program devoted to discussing the TV adaptation of his books.

Dystopian fiction, as Jill Lepore recently chronicled in this magazine, goes back a ways, but the Tripods Trilogy was arguably the first example of that later, and now very familiar, subgenre, the young-adult dystopian series, and it anticipates all sorts of details and plot points from subsequent, better-known works. Its “capping” ceremony foreshadows similar puberty-triggered procedures forced on the kids in Scott Westerfeld’s “Uglies” series, and in Philip Pullman’s “His Dark Materials” trilogy. Will must win a kind of teen Olympiad, à la “The Hunger Games,” in order to enter the City of Gold and Lead. And a game that aliens play inside their city, called Sphere Chase, seems close kin to Quidditch from the “Harry Potter” books, albeit minus the broomsticks.

Rereading the book today, though, what’s most striking is its ultimate lesson. At the end of the series, after our young heroes have defeated the Tripod rulers, Will notices troubling political developments: renewed tribalism, authoritarianism, and nationalism among some members of the resistance. Since the Presidential election last November, many people have drawn on dystopian fiction—“The Hunger Games,” in particular—to explain our predicament. The implication sometimes seems to be that we can restore democracy by rising up to defeat our newly elected leader, Katniss Everdeen-style. The Tripods Trilogy makes the case that sustaining democracy is not so simple.

Most of Youd’s books are out of print, but his literary estate, which is run by two of his children, has been working for the past few years to salvage them from oblivion with smart new editions. The most recent of these is the marvellous “The Possessors,” from 1964, a moody, creepy horror-thriller that’s of a Cold War piece with “Invasion of the Body Snatchers” or “The Thing from Another Planet.” Christopher’s best known work aimed at grownups is “The Death of Grass,” a 1956 novel that unfolds in a near-future, post-apocalyptic Great Britain after a virus kills off most of the plants on the planet. (It was made into a terrible, “Mystery Science Theater”-worthy film in 1970, called “No Blade of Grass” after the book’s title in the United States, where it was first published over several issues of the Saturday Evening Post.) Supplies becomes scarce, gangs become common, and might makes right becomes the law of the land—think “The Walking Dead” but with food shortages instead of zombies. Along with Christopher’s “The World in Winter,” from 1962—published as “The Long Winter” in the States—it deserves recognition as a key antecedent of climate fiction, or cli-fi, another of our new century’s more notable genres.

Still, Christopher’s legacy surely rests with his Y.A. fiction, the Tripods Trilogy most of all. And, while the books continue to be read (SYLE will be releasing a new U.K. paperback edition of “The White Mountains” in August), one significant obstacle to a broader cultural resurgence for the trilogy is the dearth of any characters of color and the near total lack of important female characters. Shortly after Christopher’s death, the journalist Torie Bosch described, for Slate, the experience of rereading the books as an adult and noticing, for the first time, their casual sexism and occasional racial insensitivity. Christopher mitigated this shortcoming, but only somewhat, in the series’ prequel, which includes several women, young and grown, among its main characters. The book concludes with Laurie, its male British hero, coming to realize that Hanna, a smart young woman new to his group, might make a great leader for the nascent resistance.

The need to sustain the struggle, even after apparent victory, is the note on which the trilogy concludes. “Are you ready for a new fight?” Will is asked by one of his friends. “A longer, less exciting one, with no great triumphs at the end?” Will agrees to stay connected with his friends and to fight, now, toward the never-ending goals of democracy and peace. The moral of Christopher’s story is that it’s only after monsters and Masters are defeated that the real hard work begins.