A far-future thriller combines riveting adventure and masterful world-building with profound undertones. Finn cannot remember anything before awakening in the vast sentient prison called Incarceron, but he is sure that he comes from outside its hellish confines. Claudia has known nothing but luxury as the daughter of Incarceron’s Warden; but she dreads her imminent marriage to the caddish prince of the Realms, which are trapped in a static reenactment of a pre-technological past. In parallel narratives, each discovers a chance of escape in matching crystal keys. Their separate quests gradually intertwine with increasing suspense, cresting in a series of shocking reversals and revelations. Claudia and Finn and their assorted companions are complex and comprehensible, engaging reader sympathies even as they mislead and betray each other. Elegant prose and precisely chosen details deftly construct two very different worlds, hinting at layers beneath the glimpses the tale permits; attentive readers will hear echoes of classic tales, resonant with implications about the meaning of stories, of faith and of freedom. Like the finest chocolate, a rich confection of darkness, subtlety and depth, bittersweet and absolutely satisfying. (Science fiction. YA)