1. D. Kahn, The Codebreakers: The Story of Secret Writing, Macmillan, New York (1967).
2. There are several good textbooks on cryptology where the modern techniques are explained, my favourite titles include: D. Welsh, Codes and Cryptography, Clarendon Press, Oxford (1988).
3. B. Schneier, Applied cryptography: protocols, algorithms, and source code in C., John Wiley & Sons, New York (1994).
4. G. Brassard, Modern Cryptology: A Tutorial, Springer-Verlag, Berlin (1988).
5. D.E. Denning, Cryptography and Data Security, Addison-Wesley (1982).