Flatland: A Romance of Many Dimensions

Edwin Abbott's Flatland is a short and fun novel that imagines two-dimensional worlds inhabited by triangles, circles, squares, points, and other (personalized) objects. It's not very deep or complex — or at least the parts of the book that attempt to be deep are the least successful — but it's a useful introduction (especially for younger readers) to the nature of dimensions and even space-time/relativity.