What began as outdoor survival here turns eerily psychological as Cross wraps up this grippingly original series. Down in the cavern, Zak tells of a girl stolen from her protective father by wicked robbers. Lorn balks at this stunning untruth: She was that girl, forced to live in a dark hole under the floor, and the robbers were her rescuers. But how does Zak know her story at all? Aboveground, in the normal-size world, Lorn/Hope’s deranged mother and brother Warren pursue the “robbers.” Frail Warren narrates sometimes, showing the tragic delusion created in this abusive family. Tom encounters the blue-eyed man who understands the shrinkings, then begins to sustain bruises from being crushed internally by the world’s emotional pain. Robert and Emma are desperate to understand, but only Tom acquires a terrifying new power. Profound moral ambiguity and spiraling human pain lie at the heart of Cross’s plot, which offers no neat redemption but partial resolution, even between Hope and her father. Readers should start with the first installment to absorb this grimly mesmerizing and fascinatingly unusual series. (Fantasy. YA)