In the distant future human civilization is clustered around the great city of Spearpoint, built on the surface of a huge spire extending up into the heavens. The laws of nature are not constant, with Spearpoint divided into different zones where different levels of technology are possible, from the ultraadvanced heights of the Celestial Levels at the top to Horsetown, were even simple mechanical mechanisms break down. Humans can live in any of them, but leaving your native zone without medication or an unusually adaptable constitution is dangerous due to “zone sickness,” a painful and potentially lethal condition that occurs as your body is forced to cope with natural laws subtly different from what it's accustomed to.
The main character is Quillon, a pathologist working in the mid-level Spearpoint community called Neon Heights. The story begins when he is presented with a dying “angel,” a transhuman resident of the Celestial Levels who fell to Neon Heights. Before dying, the angel delivers the message to Quillon that was the real reason for its “fall.” He knows that Quillon is an angel as well, the last survivor of a project to reengineer natives of the Celestial Levels to survive and function in lower zones- and now one faction among them is looking for him.
Quillon has no desire to go back and seeks out aid from Fray, an ally in Neon Heights' criminal underworld who has helped him keep his real identity hidden. Fray introduces him to Meroka, an expert in covertly getting people in and out of Spearpoint. They flee the city, pursued by angels with far cruder versions of Quillon's adaptations, and escape into the world beyond. But there is no safety to be found in the world outside Spearpoint- a cold, hostile place with its own zones on a much larger scale, where bands of bandits and psychopaths prey on the sparse population, cyborg monstrosities harvest living human body parts to replace their own, a militaristic, nomadic society called the Swarm rules the sky from an armada of dirigibles, and zone boundaries can shift unpredictably.
But soon there is no safety to be found in Spearpoint, either, when the world is unexpectedly struck by a zone shift of unprecedented severity, this time affecting Spearpoint itself- as Quillon realizes, watching the great glowing spire of his distant home, as its lights start to go out...
I enjoyed Terminal World. The plot had no trouble keeping my interest, and way it unfolds as the story of Quillon's escape becomes part of much larger events is effective and Quinlan himself is a compelling character.. The story's revelations about its world's history and true nature are interesting and well-paced, and the reasoning eventually given for why things work as they do makes sense and doesn't feel too handwavy or arbitrary.
The central premise of the Zones is intriguing, as is the strange hybrid setting Reynolds creates with it. Travel between zones is extremely difficult and dangerous, so much so that even with sufficient “antizonal” drugs people who do so frequently eventually develop mental problems as the accumulated stress of too many transitions starts to take a neurological toll. There's a painful feeling of division and separation throughout the setting. People in the lower levels of Spearpoint and its surrounding lands look up at the electric lights of of far wealthier societies that are mere miles away but forever denied them, and can spend their life savings for a short once-in-a-lifetime excursion upwards for mundane but life-saving medical procedures.
The fact that it also provides a justification to have “angelic” transhumans, an ancient structure that reaches beyond the atmosphere, and cyborgs that hunt people for their organs in the same setting as aerial battles fought by giant steampunk dirigibles is also pretty cool, naturally.
Speaking of which, the “vorgs,” or carnivorous cyborgs, that live beyond Spearpoint are especially interesting. Vorgs are ancient intelligent machines that have survived by replacing many of their original components with living matter better able to adapt to the lower zones. They're absolutely horrifying- grotesque, predatory, intelligent but seemingly possessed of no desires or drives other than survival, utterly without conscience- and simultaneously the most pitiful figures in the story.
Quillon is an interesting protagonist, and I enjoyed seeing how he developed. He's a cold, isolated, seemingly callous man by necessity rather than nature, after spending years in hiding and exile, and the events of the story affect him in ways I found both compelling and believable.
I'd recommend Terminal World for fans of science fiction, as well as any steampunk fans interested in a story that mixes in science fiction elements or has a setting different from the usual alternate history Victorian trappings of the genre. If you're a fan of Alistair Reynolds' more conventional science fiction works like Revelation Space or House of Suns, I'd encourage you to give this a chance even if you're not interested in steampunk; it's still very much the sort of story and worldbuilding you'll probably like if you enjoy his other work. It's an all-round strong book with plenty to offer lovers of both genres.