I remember reading this out of curiosity about 3rd grade, in the US about 8-9 years old. I read the sequel right after, enjoying that too: https://en.wikipedia.

I remember reading this out of curiosity about 3rd grade, in the US about 8-9 years old. I read the sequel right after, enjoying that too: https://en.wikipedia.org/wiki/Sphereland https://en.wikipedia.org/wiki/Sphereland It triggered a life long interest in geometry and classical philosophical documents.