I can second the recommendation for "Flatland." The full title is "Flatland: A Romance of Many Dimensions." It was written in 1880 by Edwin A. Abbott. Amazon.com has a Dover paperback edition. I read this several times when I was younger and found it enjoyable on many levels. It not only treats with the different mathematical dimensions, but also lampoons the Victorian class system and our difficulty in believing anything we haven't seen with our own eyes. It's a short read and a lot of fun.