All fiction begins with the question "what if?" -- as in "What if a bunch of pilgrims set out on the road to Canterbury and told stories along the way?" or "What if a governess discovered that her handsome, brooding boss has stashed his crazy wife in the attic?" or (unfortunately) "What if Adam Sandler got his hands on a remote control that could fast-forward time itself?" Speculation is at the heart of the enterprise, and sci-fi and fantasy are merely its logical outcomes. That's why they're often grouped under the term "speculative fiction," or "spec-fic." The work of the writer's imagination is always visible to the reader, no matter how traditional the storytelling. And the best spec-fic results when that imagination works in hyperdrive, launching from a clever premise and shooting for the next galaxy. As lifelong fans already know, this is a true literature of ideas, investigating not only physics, biology and technology but also society, community, government -- even (and often) race and gender.

Mechanical Moon Take the work of Alastair Reynolds, a former astrophysicist who has published seven novels. In the latest, Pushing Ice(Ace, $25.95), a comet-mining expedition is sent to examine a moon of Saturn that has begun to behave strangely. The "moon" turns out to be an alien machine, hurtling through space and snagging the mining ship in its wake. The crew members find themselves making the most significant journey in human history, sending back data that will massively accelerate scientific progress on Earth. But there's a catch: They'll never go home again.

Reynolds has chosen a classic space-opera plot: People are plucked out of their native environment and forced to make a new life, far away. In this case, the span of time as well as space is breathtaking: While the main story begins in 2057, with Capt. Bella Lind and her crew rocketing out of our solar system, the book also involves characters born an astounding 18,000 years later. The laws of relativity allow these people to meet across millennia; meanwhile, a life-extending device means that we can follow the crew members over many decades, as they move from the challenge of basic survival in an alien landscape to the creation of a new society. Reynolds crafts a devastating sense of isolation, culminating in the last pages when Lind witnesses proof of humankind's desperate insignificance.

Chipped Children

Kit Reed's ideas, in contrast, lie all too close to home. The Baby Merchant(Tor, $24.95) takes place in the not-so-distant future, when infants (biological or adoptable) are scarce. Enter Tom Starbird, the merchant of the title. "Think of me as the provider," he explains. "I am an expert technician in a volatile medium, which means close attention to every detail." He finds "product" (children he deems unwanted by their birth parents) and delivers it to wealthy "clients." This clinical detachment allows him to ply his creepy trade -- until, that is, he accidentally "interface[s] with the mark," his latest "supplier," a printmaker who has gone ahead with an unintended pregnancy. This single lapse -- meeting her in a Florida supermarket -- will mean his undoing, but it may also serve him a scant teaspoon of redemption.

Starbird preys on children who haven't been "chipped": By law, all newborns are implanted with an identifying microchip pressed into their fontanelles (thanks to "Homeland Security" and "No Child Left Behind" legislation). It's a way to Lojack kids, much as we already do cars and pets, though Starbird suspects the government will use it for darker purposes. Reed, a fierce thinker, is poking at the soft spot of our national anxiety about every aspect of making babies and raising children.

Magic Metals Brandon Sanderson has come up with his own neat idea: "allomancy." Not to be confused with "alomancy" (the tossing of salt grains to tell the future), allomancy allows gifted characters in his second novel, Mistborn: The Final Empire(Tor, $27.95), to "burn" metals -- that is, access the properties of bits of metals they have swallowed. Different metals correspond to different supernatural powers -- burning tin enhances the senses, for example, while brass can be used to rile people's emotions. Only a rare few allomancers -- the Mistborn -- have the ability to work all of the metals; most can use only a specific one.

Sanderson's story takes place in a dystopia of haves and have-nots, ruled by a mysterious false god and his terrifying minions. Vin is a young street thief, running with a rough crowd. A charismatic revolutionary recognizes Vin's nascent abilities as a Mistborn and recruits her for a seemingly impossible mission: bringing down the "final empire." Sanderson's characters aren't particularly well-developed, and the allomancy sometimes feels a little like a video game trick (press X-Y-X-X to burn steel!). But he has created a fascinating world here, one that deserves a sequel.

Wonder Women

Ideas are paramount in Daughters of Earth(Wesleyan Univ.; paperback, $24.95), an anthology of 20th-century feminist science fiction. Editor Justine Larbalestier of the University of Sydney has paired each of the 11 tales -- from a 1927 "scientifiction" (the early term for the genre) to a 2002 story from -- with critical commentary. But don't let the academic slant obscure the fact that these are mostly really good stories, some of which have been out of print for decades. Octavia Butler's 1987 "The Evening and the Morning and the Night" is included, and it is a great pleasure; alas, Ursula K. Le Guin isn't represented at all (this is partly, but unsatisfyingly, explained in the introduction). While the essays offer context and history, you'll look to this volume for the storytelling, first and foremost: If you're going to read about big ideas, you might as well enjoy it. ·

Sara Sklaroff is a Washington-based writer and editor.