The Code Book is one of the few books I have purchase in a high street bookstore this decade! I remember not having a book to read for my commute home so I ran up to Borders (whilst it was still solvent and eventually picked this one after perusing the computer book section for most of my lunch!). The content can be considered quite dry as it is a book on cryptography, but Simon Singh is a very engaging author and he presents the content in a readable manner.
The Code Book starts with Mary Queen of Scots losing her privacy (and a lot more besides) then meanders through the centuries demonstrating how ancients guarded their messages with crude ciphers easily broken but eventually hardened then broken again in a constant battle between those with secrets and those who wish to pry.
There are several detours into how long forgotten scripts were deciphered through comparison with known languages (ancient Hebrew & Greek were the keys to understanding hieroglyphs). The content is somewhat out of date given the current breakneck speed of crypto, but as an introduction to the technology it still serves the content well.
The section about cracking the enigma is fascinating; Poland, having a greater need to break the ultimate encryption machine of the day was driven to succeed…right until the encryption level was increased just prior to the German invasion of Poland. The book progresses through to the RSA, Alice, Bob and public keys, with some consideration of quantum computing. Not bad coverage for a 10-year-old book.