This 1884 novella was written as a satire on the social hierarchy of Victorian culture. The noteworthy part is that it takes place in a two-dimensional world occupied by geometric figures, line segments, and regular polygons.
The narrator, a "humble Square" is visited by a three-dimensional Sphere who introduces him to the world of Spaceland. Once his mind is opened to the possibility of new dimensions, he tries to convince the Sphere of the theoretical existence of a fourth spatial dimension. Offended, the Sphere returns him to Flatland in disgrace.
I really wanted to like this book. The concept is so nerdy that it's awesome, and there are all these great mathematical line drawings throughout. But when you come down to it, I'm really just not a math person.