The 26th installment in Terry Pratchett's popular Discworld series of fantasy-satire novels.
Briefly, the plot concerns the Auditors, who are something like the Universe's middle management, and who want nothing more than to rid the world of its sentient life forms, which they regard as untidy and unnecessary. The Auditors have appeared previously in the novels with various schemes to accomplish this end, and this time they have contracted with the Discworld's foremost clockmaker to produce the Perfect Clock, which will stop time and bring Life As We Know It to a grinding halt.
Ultimately, the task falls to the History Monks (whom fans of the series will remember from Small Gods) to foil the plan and save humanity (and dwarfdom, and trollhood, and so forth). The book also features many of the Disc's other familiar characters, often in new roles: Death, his granddaughter Susan, the Death of Rats, the monk Lu-Tze, one of the Igors, the Four Horsemen of the Apocalypse - and the fifth horseman, who quit the group before they were stars.
Editorial Note: It is amusing to compare and contrast Pratchett's apparent view of time and its impact on our daily lives with that of Kurt Vonnegut, as in his final novel Timequake. Both writers are among the classic satirists of our era, somewhat alike but also utterly dissimilar - it's a little like the difference between Star Wars and Star Trek.