If anyone is interested in the history of codebreaking, i can definitely recommend 'The code book' by Simon Singh
(ISBN 1857028899). It is an easy to read and fascinating book on how encrypters and code breakers have battled each other through history. I found the chapters on egyptian hieroglyphs and the WW2 chapters (with the infamous enigma machine) awesome.
But this is not all: the book also contained a £10000 crypto competition. He left 10 messages, in increasing difficulty of encryption (without revealing which encryption was used), at the back of the book, to be cracked by those who had the courage AND the knowledge. It took about a year before one group of Swedish enthousiasts had uncovered all 10 messages, the last one being an RSA encrypted string they attacked in true SETI style. You can read their story