[jimsmart]

"Applied Cryptography" by Bruce Schneier, is 'the bible', covering the motivation behind the invention of many of modern algorithms, as well as most of the maths.

"The Code Book" by Simon Singh is an enjoyable read regarding the historical aspects of codes, ciphers and cryptography. I don't know how this compares with other books on the same subject, but I found it interesting and informative.

It's not in the same league as Applied Crypto though — totally different kinda thing: The Code Book is something you could gift for xmas, whereas Applied Crypto is a hard core coder reference book.