Confusion
Because both ciphers classically employed novels as part of their key material, many sources confuse the book cipher and the running key cipher. They are really only very distantly related. The running key cipher is a polyalphabetic substitution, the book cipher is a homophonic substitution. Perhaps the distinction is most clearly made by the fact that a running cipher would work best of all with a book of random numbers, whereas such a book (containing no text) would be useless for a book cipher.
Read more about this topic: Running Key Cipher
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“The small force that it takes to launch a boat into the stream should not be confused with the force of the stream that carries it along: but this confusion appears in nearly all biographies.”
—Friedrich Nietzsche (1844–1900)
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—Bible: Hebrew, Deuteronomy 28:28,29.