I've seen ciphers (usually in spy drama shows) that involve taking a book and writing down an index to individual characters. Essentially it's a keyed substitution cipher, where the key is the name and exact edition number of the book.
Imagine a book that has 500 pages, with approximately 60 lines per page and 80 characters per line. We might write
238:4:64 to specify that we're on the 238th page, 4th line, 64th character. A five character message might be encrypted into
114:25:22 374:41:46 182:17:62 63:53:8 50:8:18.
Without knowing what the book is, I would imagine this is provably secure as long as the person encrypting never re-used an index. Am I correct in this assumption?
For fun, you could try this out:
12:9:3 183:15:7 238:14:11 310:9:38 194:11:17 7:1:1
I'm using a
page:line:character format, ignoring titles, spaces and punctuation. The book is Charles Darwin's On The Origin of Species, 6th edition.
N.B: I'm not trying to get you to crack this, just adding it because it nicely demonstrates the way this works :)