Sapphique by Catherine Fisher

Finn has escaped Incarceron, but Keiro and Attia are still Inside. Outside, things are not at all what Finn expected - and both Finn's and Claudia's very lives hang on Finn convincing the Court that he is the lost prince. Back Inside, Keiro and Attia are on the hunt for Sapphique's glove, which legend says he used to escape. In order to find it, they must battle the prison itself. Incarceron has built itself a body and it wants to go Outside - just like Sapphique, the only prisoner Incarceron ever loved. "High-intensity, mind-bending . . . Fisher further explores themes of reality, illusion, and freedom without losing her intensely original world-building and authentic characters." - Booklist, starred review "Even as the steadily ratcheting certainty of impending catastrophe keeps the pages turning, the sheer richness of the evocative descriptions demands that every sentence be savored. . . . For those who can appreciate the interplaying reflections of lies, myths and memory, a modern masterpiece." - Kirkus Reviews, starred review

This book is part of the "Incarceron" series.
Here are some other books from this series: