The Zodiac may have drew inspiration from David Khan's Codebreakers book released to the public in September 1967 and the answer to the infamous 340 Cipher lies somewhere within these pages. David Khan himself would be the successor to many other books, citing 'The American Black Chamber' as the most famous book on cryptology ever published, a book that sparked huge public interest in the so called Black Chamber that existed in the United States at the time, and not uncommon to many countries who formed these clandestine code breaking groups.
Herbert Yardley (1989-1958) dismayed at the vulnerability of American coding, became integral to the newly formed military intelligence unit MI-8, becoming key to the solving of a cryptogram hidden in the sleeve of Lothar Witzke, a German spy, arrested on the Mexican border on February 1st 1918 and later sentenced to death for espionage, however this was later commuted to life imprisonment at Leavenworth Prison. Lothar Witzke, aided by German pressure was eventually released and pardoned in 1923.
Herbert Yardley was also instrumental in cracking Japanese codes, effectively restricting them on the naval front, along with other mail intercepts, but with the appointment of Henry Lewis Stimson, the Secretary of State under Hoover's governmental reign, the days of MI-8 were numbered, eventually ceasing operations on Halloween Day 1929.
Two years later he released his critically celebrated memoirs 'The American Black Chamber', though the book failed to impress the United States government, as it made uncomfortable reading, bearing in mind the analysis and code breaking implemented to hijack other countries 'traffic' of sensitive material.
Zodiac Killer Ciphers is an excellent website providing copious tools to examine the Zodiac ciphers, including Webtoy to investigate further the 340 Cipher mailed to the San Francisco Chronicle on November 8th 1969.
There is also another high quality website deriving its name from the clandestine code breakers mentioned above, called Black Chamber, providing a host of information explaining the workings of different cipher types and allowing you to enter the text you would like to experiment with, after all it is likely the Zodiac used one of them. The hard part then begins, find the likely path the Zodiac chose and find the key he used. It could well be sitting in plain sight within his numerous letters or cards, the problem being, when it is staring you right in the face it is often harder to see.