With its fairly famous opening line — ‘I had reached the age of six hundred and fifty miles.’ — I kind of thought I had read this before, long ago, maybe as a teenager. But no. It turned out definitely not.

A young man learns his place in a city — Earth City, as they call it — which is moving. Rails are placed before it and lifted up behind it so they can be laid in front again. The city is winched along on the rails by fits and starts. Why? Why is it in motion, and why do the inhabitants work desperately to keep it so? And why is the fact kept hidden from city dwellers who are not ‘guildsmen’?

The answers, or some of them, are within. Though there is no answer to why ‘guildsmen’ is the correct word. Women are second-class citizens in the city. And worse outside it, on the whole.

The people of the city are human, they speak English mostly. They know their ancestors were from Earth planet, as they refer to it. The people in the villages they pass also seem to be human, and they mostly speak Spanish. The sun appears not as a sphere, but as a kind of disk with spikes top and bottom. What can be going on? The title suggests some kind of inversion, but what is it?

Ultimately the mystery isn’t solved in a very interesting way, and the ending is sad, but maybe happy, but maybe sad.

Priest has written much better books, but it bears reading.