The Mathematical Theory of Communication by Claude E. Shannon and Warren Weaver

Shannon and Weaver's book is a classic piece of science, and surprisingly readable at that. Weaver's introduction reads almost like popular science, and balances Shannon's mathematical (yet very clear and easy to follow) description of communication.

Most of us working with computers these days studied this theory in undergrad, not to mention that we probably specialised in one of the many branches of research that sprouted from it. However it is very rewarding to read the original theory.

The message is very clear that close to the source.