Terry Pratchett's Night Watch is the latest installment in his Discworld series, and it's a good 'un.
The Discworld series is actually made up of several subserieses -- the Witches books, the Rincewind books, the Death/Susan books, and (the subseries in which this book is classed) the Watch books. Night Watch is not an ideal place to start reading Pratchett, as it assumes significant familiarity with at least the earlier Watch books -- which is a bit of a pity, really, because I think this is one of the best Watch books Pratchett has written.
I was a bit wary at first, because the plot seemed frighteningly cheesy: A lightning bolt causes a temporal rift into which Vimes (and a criminal he's struggling to apprehend) fall; they end up travelling back in time to Vimes's early days in the Watch, where changes are made to the time-space continuum, which Vimes needs to put right. I think I've seen that story on Star Trek a few dozen times already.
But here, that's almost just a framing device. I mean, it's an important part of the plot and all, but it's definitely not the entirety of the thing. The book is really about corruption, revolution, and experience, and it amply displays Pratchett's greatest strength, which is his humanistic understanding of people and societies. This is the sort of book that's much better than the plot synopsis.
If you've read the other Discworld books, be assured that this is a significant and worthy entry into the corpus; if you haven't, go read Small Gods and get back to me later.