Security
Essentially, the code version of a "book cipher" is just like any other code, but one in which the trouble of preparing and distributing the codebook has been eliminated by using an existing text. However this means, as well as being attacked by all the usual means employed against other codes or ciphers, partial solutions may help the cryptanalyst to guess other codewords, or even to completely break the code by identifying the key text. This is, however, not the only way a book cipher may be broken. It is still susceptible to other methods of cryptanalysis, and as such is quite easily broken, even without sophisticated means, without the cryptanalist having any idea what book the cipher is keyed to.
If used carefully, the cipher version is probably much stronger, because it acts as a homophonic cipher with an extremely large number of equivalents. However, this is at the cost of a very large ciphertext expansion.
Read more about this topic: Book Cipher
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