I seem to remember a Sherlock Holmes story where two people exchange information in the following manner:
They each had a copy of the same book
The author of a message would find each word somewhere in the book, then write down:
- The page number
- The number of the word on the page (first, second, third...)
Impossible to break without a copy of the book
Sherlock had intercepted a coded message, and was able to read it - but only by guessing which book had been used, knowing what he did about the author!
But I can't remember which story it was...