Thief Of Time

'Some humans would do anything to see if it was possible to do it. If you put a large switch in some cave somewhere, with a sign on it saying "End-of-the-World Switch. PLEASE DO NOT TOUCH", the paint wouldn't even have time to dry.'
Time is a resource. Everyone knows it has to be managed. You can let it move fast or slowly, but what you mustn't do is allow it to stop.

The Monks of History have the glamorous job of time management in the Discworld. But the construction of the world's first truly accurate clock starts a race against, well . . . time.

Monk Lu Tze, his talented apprentice Lobsang Ludd and Death's granddaughter Susan must stop the clockmaker before he completes his task.

Because a truly accurate clock will stop time. And when time stands still, everything in existence stops with it. Then, there really is no future . . .

'This is the best Pratchett I've read' Sunday Telegraph
Thief of Time is the fifth book in the Death series, but you can read the Discworld novels in any order.