Catherine Fisher brings to a conclusion the story started with Incarceron in the same imaginative and gripping style as she has displayed in her best work.
Fisher's discussion of the meaning of "freedom" might be thought-provoking amongst her youngest readers, others will probably have considered the question for themselves or come across the ideas else-where. That said, I have not experienced such an entertaining work dealing with the topic before.
I have a niggle regarding the pair of books taken as a whole: An event occurs "off-screen" in Incarceron that is inexplicable in the terms set out by the books. Unless I have missed something, this appears to be a significant plot-error! I refrain from detailing it here, so as to avoid spoiling others' enjoyment and hope to contact the author regarding it.