Cryptography/Beale cipher
The Beale Cipher is a cipher in which two parties agree on a key which is a text (e.g., The Declaration of Independence which was used by Thomas Beale[1] as the key for one of his three encrypted texts), and the words in the text are then enumerated, and the encrypted text consists of numbers from the key. The numbers will then be replaced with the first letter of the word from the key-text when the cipher text is being deciphered.
The origin of the cipher was that Beale left an encrypted text with notes where to find his gold (worth $20 million, [2]), although many commentators believe the story about the hidden gold to have been a hoax.
There are no short cuts to break this cipher like there is for Vigenère, the mono-alphabetic or the polyalphabetic cipher; ultimately, the only way to successfully decipher it is to guess the original key-text, which may not be an easy task. The difficult depends on clues left in the cipher text. For example, it may be possible to infer the length of the book, etc., from the cipher text.