Terry Pratchett 
Thief Of Time 
(Discworld Novel 26)

»The universe is, instant by instant, recreated anew . . . The perfect moment is now. Be glad of it.»
Time is a resource. Everyone knows it must be managed. You can let it move fast or slowly, but what you mustn»t do is allow it to stop.
On the Discworld, the History Monks have the important job of ensuring that tomorrow always comes. But the construction of the world»s first truly accurate clock starts a race against, well . . . time.
Monk Lu-Tze, his 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 completely.
And then, there really will be no tomorrow . . .
»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.