(posted by Kristycat)
A book code is an old way of sending secret messages. See, normally to send a secret message, you need either a cipher or a codebook – but ciphers can be broken, and codebooks can be stolen (and are pretty incriminating anyway – a bad thing to be caught with if you’re a spy!)
So instead, you use a regular book – something innocuous, that both you and the person receiving the message could own without it looking strange and suspicious. Traditionally, you would use either a dictionary (because it has just about all the words you could realistically need) or a Bible (because for large swaths of history in certain parts of the world, just about everyone could be expected to own one), but really as long as the book is big enough to have a good selection of words and as long as you and your friend both have the same edition, you could use anything!
The basic version is this: write down the page number on which the word is found, then write down the number showing which word on the page it is. So if you want to write “Hello,” you’d find a page with the word “hello” on it – let’s say page 53 – and then count the words on the page. If “hello” is the 32nd word on the page, you would start your secret message with “53 32” – “Hello!”
At that point of course you can ALSO employ codes and ciphers and layers upon layers of encryption, but it’s a decent little code just to start with.
This has been your fun fact of the day! Go forth and send secret messages!