Flatland: A Romance of Many Dimensions. This is a very strange and interesting story. Written almost 250 years ago, it takes place in two-dimensional space which makes it challenging to even consider at first. Once you get the idea, it explores many interesting sociological concepts from class mobility to gender.
The two-dimensional nature of the characters and plot leads to many odd discussions. For example, class comes from the number of sides a shape has. A square is of higher class than a triangle and so on. Children rarely rise to a higher class than their parents, and it takes many generations to achieve higher class.
A triangle is distinguished by the acuteness of its smallest angle. Pointier triangles are a natural weapon, and best suited to fighting-jobs like policing or soldiering.
This will be a difficult book to recommend. I think it will have to be a very particular sort of person who I will one day recommend this to.