[kennethn]

I also recommend A.K. Dewdney's The Planiverse, published in 1984. It tells the story of computer science students who communicate with denizens of a 2D world. The diagrams and graphics are delightful.

I read it when it came out (I was 13) and it's one of the reasons I entered computing.

http://en.wikipedia.org/wiki/The_Planiverse