Ruby and Clancy use a special code called a Vigenere Cipher. They leave each other codes in the knot of an oold oak on Amster. The books give you clues to the keyword of the codes, and here they are:
Look Into My Eyes clue: It appears in the name of Ruby's favourite Diner. It only appears *ONCE* but it means
Take Your Last Breath clue: It swims in the sea, but it's not a fish. It was once on the surface, but now it's on the bottom...
Catch Your Death clue: This royal personage's suggestion for survival was to let them eat cake.
Feel the Fear Clue: This is something you might do to a pesky fly.
If you cannot work out the keywords, here they are (I worked them out myself :-))
Look Into My Eyes: Double
Take Your Last Breath: Sea Horse
Catch Your Death: Marie Antoinette
Feel The Fear: Swat
If you want to crack the code from the book, go to http://rubyredfort.com/rubys-room/code-breaker/crack-code/ or to find out how to create your own version, go to http://rubyredfort.com/rubys-room/code-breaker/explanation/