Much easier to remember than random substitution ciphers, and therefore more practical for spies, these ciphers start with a keyword, say TUESDAY, then continue alphabetically, omitting all repeated letters, until every letter has been used.
To avoid confusion, use lower case for the plaintext (which is what the message to be encrypted is called) and capitals for the cipher or code.
To use this code, a spy would simply have to remember the key TUESDAY and know the rules for constructing substitution ciphers.
What is her message to you?
IDQDM MDQDTG WJPM FDW OJ TIWJID DVEDKO WJPM AMBDISN
Why not compose your own substitution cipher, using your own key? (Remember to omit any repeated letters in the key). Or try to crack this: