At first glance, you might think that a book about people, trolls, dwarves, wizards and mysterious monks would fall neatly into one of the various realms of the fantasy genre. Instead, _Night Watch_ combines a fantasy/sci-fi setting with a hard-boiled police drama, a coming-of-age story and social commentary, including characters to rival Charles Dickens. Pratchett weaves a complex, inventive plot through time-travel, myth, mysticism, and a gritty sixteenth-century urban landscape reminiscent of Dickens' London or Hugo's Paris. He finds time and room for a degree of comic relief, but returns, as in all his Commander Sam Vimes novels, to the central issue of keeping the inner darkness at bay, even under crushing circumstances.
This is one of Pratchett's more-recent books, and in it we see the master story-telling delivering a richness of character and plot that is as satisfying as a good red ale. I recommend this book to anyone with a taste for compelling characters in complex plots. It may not be the best place to enter the Vimes saga (Pratchett tells the story of his master cop/everyman in half a dozen or more of his novels), although it is the only glimpse we've gotten so far of the early career of the young Sam Vimes, and of the youth of Lord Vetinari, the Patrician of the city.
If you wanted to start with the beginning of Vimes's character development, the much-earlier (in publication date) novel _Guards! Guards!_ takes him from the gutter to the beginnings of self-respect and the beginning of his left-handed romance with Lady Sybil.Ramkin.