Night Watch is the story of a man who travels back in time on a flat world where magic works. But it is a story of a very real-world theme: the maintenance of one's own integrity.
Sam Vimes is the Commander of the Watch of Ankh-Morpork, Discworld's largest city. As he pursues a criminal, a magical accident hurls both him and his quarry back several decades. He talks his way into a sergeant's post on the Watch, and takes his younger self under his wing. It is a period of major civil unrest and civic corruption, and Vimes finds that there are times when he must just "do the job in front of him," and times when he must refuse to do it.
Like other great humorists, Pratchett at his best is both funny and extremely serious. We find humor in the obstinate way that many of the characters cling to their preconceptions, but the fact that they do this leads to armed conflict and to people's deaths. The book makes a number of allusions to Les Miserables, but unlike some of Pratchett's other books, it is not a send-up of any single work or genre.
Some of the issues which the book touches on are very relevant to the world of the twenty-first century. Among these are whether it is all right to look the other way when turning prisoners over to torturers, and whether to kill a person who eminently deserves it rather than bringing him in for trial.
Regular readers of Pratchett will enjoy seeing younger versions of familiar characters. Libertarians will enjoy lines such as "Taxation is just a sophisticated way of demanding money with menaces," as well as scenes such as a socialist revolutionary's attempt to convince a cobbler that he'll be much better off when his property is taken away from him. But the heart of the story is a man determined to do what is right in a difficult situation, superbly presented. I regret that I had already sent in my Hugo nominating ballot before reading this book.