Thief of Time is the twenty-sixth book in the seemingly neverending Discworld series of books by Terry Pratchett. Initially, these books were a parody of "Conan the Barbarian"-style fantasy, then evolved into satires about pop culture and contemporary politics, and eventually developed a peculiar knack for murder mysteries and cosmic crises of metaphysical proportions. Thief of Time is about one of these crises.
Susan Sto Helit, granddaughter (by adoption) of the anthropomorphic personification Death, has been trying to preserve her sense of humanity ever since she discovered her family history. Previously, in Hogfather, she was a governess; she's now taken the next logical step into elementary school teaching. But Death hasn't forgotten her, nor her usefulness in situations involving the Auditors.
The Auditors are grey, shapeless, nameless beings who have been harassing Death for some time now. Their self-appointed job is to keep things neat and tidy in the universe, and life in all its shapes and forms is exactly the opposite of that--unpredictable, nondeterministic, and full of intangible things like love and hope. They want to do away with it, but can't through direct intervention. And Death likes life, because without it he's out of a job. But he can't directly intervene, either. So he calls on Susan to help out.
This time, the Auditors are attempting to re-create a special clock--one that can measure the "cosmic tick" of the universe and trap Time herself within it. Stop time, and you stop change, preserving the universe in a tidy and calculable form that the Auditors can enjoy. There's even a clockmaker in exile from his Guild, a man by the name of Jeremy, who's uniquely suited to building such a device.
Susan's not the only one trying to find Jeremy before he can build the clock, though. The Monks of History, last seen behind the scenes of Small Gods, have been taking care of time for... well, it doesn't matter how long, shuttling time away from where it's not needed and adding it to where it is, all across the Disc. They were responsible for restoring history the last time a clock like this was built (and broke), and they're determined to keep it from happening again. The trouble is, it's hard to find a clock that can stop time before it's even built, and it's harder to stop it after it's started working....
Thief of Time is perfect for geeks because of its well-disguised references to chaos theory (the "Mandala" is clearly derived from the Mandelbrot set), quantum mechanics (the "cosmic tick" = Planck's constant), and the movie "The Matrix" (the monks can "slice time" to move ultra-fast, and know more bizarre-sounding martial arts than the audience at a Jackie Chan film festival). You don't have to know the references, fortunately, because Pratchett has to explain them in such a way that his own mythology can interact with it. But knowing where his ideas come from makes the book that much more fun.
The book didn't conclude as well as I'd liked, because a few of the minor characters weren't really tied up. But the major characters were all great. Lu Tze, the monk that played the important part in Small Gods, should be a character we'll enjoy seeing again in future books. Susan is spot-on her usual personality, harassing the mortals she's forced to interact with on a daily basis. Death spends some time chasing down his fellow Horsemen. And the Death of Rats is there mainly because (a) he's how Death always contacts Susan, and (b) to make a not-too-subtle "hickory dickory dock" joke halfway through the book.
Susan's first apperance in Soul Music was a bit of a disappointment, but her role here, like in Hogfather previously, is perfectly suited to her. Death is a fun character, but he lacks personality by his very nature. Susan acting on his behalf is much more enjoyable. More books with her will be welcome in the Pratchett audience, and this one's worth the purchase.
YA "let's do this better than Slashdot's editors can" Pratchett book review