A Mathematical Theory of Communication (1948) by C. E. Shannon [0][1]
Information theory: a Tutorial Introduction (2nd Edition) by James V Stone bit more modern introduction/hands-on approach.
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[0] : paper : https://people.math.harvard.edu/~ctm/home/text/others/shanno...
current book version of paper is 16th addition.
[1] : Why [0] matters : https://www.quantamagazine.org/how-claude-shannons-informati...