Stephen Baxter is a talented writer whose writing covers many differents areas of what we'd call "science or speculative" fiction. This particular book combines aspects of what I consider social science fiction with a bit of mystery. The protagonist discovers that his family has a centuries old tie with an organization and this connection is involved with his sister leaving the family at a very early age to live in a mysterious "order" or school. His research leads him to discover a group of people who, after co-existing side by side but separately from those of us in the known world, have come to belong to a variant of the human species.
This variant follows various patterns found within nature and does have a certain potentially superior survival mechanism. It is my impression that the book implies that this human variation is possibly a natural evolution of the human state - further, that it is likely that other intelligent species also could have a tendency to evolve in this manner.
For those of us who deeply enjoy Baxter's hard science fiction, this book may come as somewhat of a disappointment. I believe there is/are even (a) sequel(ae) to the book, but as the book left me a bit cold, I have not bothered to even impress the facts upon my consciousness, so I cannot truthful claim whether the concept has been extended by Baxter or not.