Katherine Hayles:
Telegraph code books, in the hundred years of their use (ca. 1850-1950), served primarily as an information compression technology, matching a phrase or sentence with a code word, thus saving money on telegram costs. On occasion, they were also used for secrecy, since typically only someone possessing the same code book as the sender would be able to decode the message. In the books’ heyday, virtually every industry had its specialized version; shipping, banking, railways, carpets, and rubber all had code books designed especially for them, as did many other industries as well. In addition, general purpose code books also flourished, many going through multiple editions. The code books were widely used, especially by businesses but also private individuals. Estimates are that 95% of all transatlantic telegraph traffic was encoded, and a high percentage of domestic traffic as well. After the telegraph became obsolete, most libraries and individuals purged the books from their collections; of the hundreds of thousands of originals, perhaps only hundreds remain. Old enough to have progressed from junk to collectible, they have now become prized items and sell for fancy prices at online auction sites.