Flatland - A Jump of Many Dimensions

From the introduction: Flatland is a world that exists on the two-dimensional plane, where its inhabitants—literal geometrical shapes—live in a highly-structured society organized into classes based on the number of sides of a figure. You’ll enter at the turn of the 2nd millennium shortly before a certain A Square is selected by a visitor from the 3rd dimension, to be his apostle and to proclaim the truth of spaceland.