The two volumes gather Friedman’s scattered publications—many originally issued as classified military memoranda, training notes, or internal Signal Intelligence Service documents. They include his work on cipher construction, cryptanalysis techniques, frequency analysis, index of coincidence, transposition systems, substitution ciphers, and early machine cryptography. Together, they form a technical and historical record of how Friedman built the intellectual framework that later enabled the breaking of systems like PURPLE and contributed to Allied cryptologic dominance. For historians, mathematicians, and intelligence scholars, these volumes are a primary source on the evolution of American cryptanalysis.