I've read a lot of crypto books, ranging from ancient history stories (in The Codebreakers, for instance) and up to the newest math-loaded textbooks (for a course I've taken at Uni), so I didn't expect The Code Book reveal new things to me.
Rather, I fondly recalled Singh's great style in "Fermat's last theorem", and decided to read this book anyway.
I surely wasn't dissapointed. Singh has this talent to excite you about science. He did it successfully in "Fermat" and repeated it in The Code Book. He explains everything very clearly, often giving excellent analogies.
But there was more to it. I did learn new things from the book ! The stories of Linear B, Navajo codes and Babbage's cracking of the Vigenere were definitely unknown to me. The explanation of quantum cryptography was a good refresher and explained things well (though, IMHO, Singh's excitement about quantum crypto's unbreakability is a bit pre-mature. after all the quantum physics barely exists for 100 years, and physical theories tend to cripple and replace one another...)
All in all, it is a great book - highly recommended - very readable, interesting and enlightening.