Whilst neither the premis of the book (humanity emerging from a computer ruled underground enclave after two thousand years) nor the denoument (computer destroying humanity for its own good outwitted by a new evolutionary trait) is original, the strength of the book lies in the sheer skill with which it is written.
Cowper builds his world with the most subtle of brush strokes and peoples it with realistic characters. He does not explain anything. Through the story of a small group of people, he reveals their world and their culture; and whilst his light is focussed on a small group, we see enough in the half lit glimpses on the periphery to fill in some gaps, but still leave us wondering.
Like his other work, this demonstrates that the best science fiction and fantasy is every bit as well written as ‘serious’ literature and deals with equally serious subjects (in this case what it means to be human).