A book cipher uses a large piece of text to encode a secret message. Without the key (the piece of text) it is very difficult to decrypt the secret message.
To implement a book cipher, each word in the secret message would be replaced with a number which represents the same word in the book. For example, if the word "attack" appeared in the book as word number 713, then "attack" would be replaced with this number. The result would be an encoded message that looked something like this.
713 23 245 45 124 1269 586 443 8 234
To decipher the message you simply count the number of words in the book and write down each one.
