Dune Player's Journal 

Even as this thought flicked through her mind, she was bending to seek out the hidden message. It had to be there. The visible note contained the code phrase every Bene Gesserit not bound by a School Injunction was required to give another Bene Gesserit when conditions demanded it: "On that path lies danger."
Secrets are the most powerful weapons in the Imperium. Codes and messages are hidden among the most innocuous glance or most casual sentence. The secrets you learn might save your life, or your House, so they are best kept careful track of.  An elegantly designed 160-page journal to record your adventures on Arrakis.