Onyx - Achillios Chronicles, #2

Taryn is mastering the ancient machines that made Achillios livable for human beings. But now he's traveled far from his home city of Alabaster. He and a team of other operators, trained to interface with the machines, need to repair the damage done by the rebels from the hidden city of Onyx. Nobody is clear on what the rebels are planning, what they want, or when they will strike next. Those rebels, meanwhile, are scattering, fearful of discovery after their two agents in Alabaster were captured.
They're planning not only to fade into the background, but to continue their urgent mission.  Onyx continues the story of Alabaster, uncovering some of Achillios' greatest mysteries. Taryn will encounter new challenges, and he'll be forced deep into Achillio's unknown and troubled past. He'll discover lost technologies and techniques, but he'll also be confronted with the terrible cost of using them - and be left to wonder if his ancestors hadn't been right when they decided to stop using the machines altogether.