About a mobile city that must tap its power from a mysterious `optimum point', which is less effective for their engines as it gets more distant. Weird distortion of the surrounding world is based on hyperbolic functions. Parts of the yarn get pretty weird, but it is still a story well told.

The author seems to change the direction of the plot towards the last third of the book but its still worth a read.

Mathematics is not central to this novel, but there's at least a fascinating idea: the story is set on a planet with a negative curvature. More precisely, it's the solid of revolution of a rectangular hyperbola spun around one of its asymptotes (the book uses less precise terminology, but it's clear anyway what it's talking about). This is revealed to be an approximation, or the planet would be infinite (since it rotates around its symmetry axis, the world "ends" before the speed of rotation exceeds that of light). The "optimum point", by the way, is placed on the vertice of the hyperbola, but since the planet's ground slides the city never reaches it once and for all, hence the need for it to move on rails.

There's a twist concerning the planet's shape towards the end of the book, but since it's a major spoiler and a bit of a letdown for math-savvy readers, I won't write about it.