The Fifth Elephant by Terry PratchettTerry Pratchett (ISBN: 0-385-40995-8)
The Fifth Elephant is a fantasy novel by British writer Terry Pratchett, the 24th book in the Discworld series. It introduces the clacks, a long-distance semaphore system.

Current read: The Fifth Elephant

I’d like to link to terrypratchettbooks·com but they have a hilarious stupid Terms Of Use about linking or even quoting that I will not support.