Shannon's original paper on the topic was written during WWII and I believe it was classified and is much more concise as an introduction. After that, he and Weaver put together the famous and much more comprehensive 1948 paper which expanded into the noisy coding theorem. Meanwhile his original paper ("Communication in the Presence of Noise") was published in 1949, possibly after declassification. I highly recommend reading it first, taking maybe an hour to read. Another terrific intro is a chapter of a book by Bruce Carlson: "Communication Systems: An Introduction to Signals and Noise..." I have a scan of the chapter linked here: https://drive.google.com/file/d/0B9oyGOnmkS7GTFlmQ2F1RWNFd28...