I love this trilogy. It is based on a tiny little change (geometry of the universe) and exploring its consequences great and small. A daydream gone as far as a PhD thesis. By explaining physics in a fundamentally different universe it helps you understand the physics in our universe too.

The hard physics is lightened by exploring some social problems (that are also remotely derived from the different physics). This again is done as seriously as in a book exploring real social problems. When you read it, it is easy to forget how fictional all the injustice is.