A fascinating and entertaining book about living on a world in a universe where people can come in different scales. Here, there are eight of them, each one half the size of the previous scale. This comes about because there are eight different kinds of leptons (like electrons) with different masses, causing the atoms they make to have different sizes. Egan explores the possibilities this difference in sizes causes to mass, biology, physics, chemistry, etc. to come up with a world where people of different sizes have learned to live next to each other.

But all may not be well. At the start of the book, a private investigator is hired to find a missing sister. His investigations would lead his to discover a secret being hidden by some people from a smaller scale. As he passes on his investigations to fellow investigators from the smaller scale, what they find would lead to the discovery of a secret that will alter the delicate negotiated balance the people of various scales have agreed to on how to share their environment. And now, the race is on to find a solution to avert a possible crisis that may involve weapons of unimaginable power being used on the peoples of other scales.

Egan expects the reader to be able to pick up the physics of the world from the situations and characters presented in the book, without much exposition being dropped in by him. If you find the consequences of living at different size scales puzzling, then this might not be a book for you. Otherwise, the reader will find living in a world where people are of different scales to be a fascinating experience.

Egan also gives a thoughtful and rational look at how people of different scales interact with each other and what can happen if people of one scale discover they have the ability to radically change the balance of the relationship between themselves and people of other scales: a rough equivalent would be people of different cultures on our world interacting with each other to share a common environment. The debates and arguments that ensue show the different possible reactions, and it is left to the reader to discover which option will be the one that will determine the future of all the people at various scales.