Great book. Abbott uses a mathematical exercise (imagining life in 2 dimensions) to pass commentary on social structure and sometimes even religion. I loved the book--the setting is creative and the concept is thorough. I am an engineer, but math is not needed to appreciate this book.
I had sort of a personal interest in reading this because my grandmother was named after one of its characters (Ustane). The novel holds up very well even by modern standards, and it's interesting to see how many conventions of the adventure and fantasy genres were established here.