Flatland: A Romance of Many Dimensions by Edwin Abbott Abbott

As the last book in the, 25 books you should read, I am going to give you an easy one. Easy as in you should be able to read this one sitting, but you'll really enjoy it and get a lot out of it. Here is a quick synopsis Of Flatland from the publisher:

The story is about a two-dimensional world referred to as Flatland which is occupied by geometric figures, line-segments (females) and regular polygons with various numbers of sides. The narrator is a humble Square, a member of the social caste of gentlemen and professionals in a society of geometric figures, who guides us through some of the implications of life in two dimensions. The square has a dream about a visit to a one-dimensional world (Lineland) which is inhabited by "lustrous points." He attempts to convince the realm's ignorant monarch of a second dimension but finds that it is essentially impossible to make him see outside of his eternally straight line.

The narrator is then visited by a three-dimensional sphere, which he cannot comprehend until he sees Spaceland for himself. This sphere, who remains nameless, visits Flatland at the turn of each millennium to introduce a new apostle to the idea of a third dimension in the hopes of eventually educating the population of Flatland of the existence of Spaceland. From the safety of Spaceland, they are able to observe the leaders of Flatland secretly acknowledging the existence of the sphere and prescribing the silencing of anyone found preaching the truth of Spaceland and the third dimension. After this proclamation is made, many witnesses are massacred or imprisoned (according to caste).

The book is a satirical look at Victorian culture, but it is as relevent today as it was then. Another great factoid about this book, Albert Einstien used to read it every year.