Thief of Time: Discworld, Book 26

Time is a resource. Everyone knows it has to be managed. And on the Discworld that is the job of the Monks of History, who store it and pump it from the places where it's wasted (like the underwater; how much time does a codfish need?) to places like cities, where there's never enough time.

But the construction of the world's first truly accurate clock starts a race against, well, time for Lu Tze and his apprentice, Lobsang Ludd. Because it will stop time. And that will only be the start of everyone's problems.