All the records of the Incan Empire were recorded on quipus, including tax records, census rolls, harvest stores, and the like. How much quipus could encode other linguistic information is a subject of current debate, given there are Spanish reports of quipucamayocs using quipus reading to relate historical events from beyond personal memory -- and it should be noted that the Incans (and their several predecessors) had no other written language. The word is from Spanish, from Quechua khipu (Cusco dialect) or kipu (most other dialects) meaning "knot". As usual, Wikipedia is incomplete but a good place to start.
I like the idea of a spy using an otherwise innocent girl's poncho fringes as a quipu to send information over the frontier.