During the last weeks, I've read the book The Emperor's Codes: The Breaking of Japan's Secret Ciphers. As you can guess from the title, the book relates to the various UK, American and Australian code breaker teams working on breaking the encrypted communication of Japan during the second world war.
There have been plenty of books about the history of breaking Germany's Enigma ciphering machine, but information on how the Japanese codes were broken so far didn't seem to be as widespread - despite the resepective archives being opened up during the last decades.
It has been a most interesting reading. As you can imagine, at that time almost nobody had a sufficient understanding of the Japanese language, not even thinking about how to encode Japanese writing into morse code.
Nonetheless, all of the Japanese merchant, diplomatic, army and navy codes have been broken during the war. And surprisingly, the Japanese never really assumed something is wrong with their actual encryption method. All they did is to replace the codebook or the additive codebook.
Also, just like in today's GSM (A5/1) crypto attacks, even back then the importance of known plaintext could not be underestimated. The verbosity of Japanese soldiers addressing a superior officer and the stereotypical nature of reports on weather or troop movements gave the cryptographers plenty of known plaintext for many of their intercepted message.
What was also new to me is the fact that the British even back then demanded that Cable+Wireless provides copies of all telegraphs through their network. And that's some 70-80 years before data retention on communications networks becomes a big topic ;)
Overall, definitely a very interesting book. I can recommend it to anyone with an interest in security, secret services, WW2 history and/or cryptography.