Night Watch by Terry Pratchett

Sam Vimes is one of Terry Pratchett's most enduring characters in the Discworld series. First appearing in Guards! Guards! as a washed up Captain of the ineffectual night watch, he has risen to be commander of the whole city watch, forming it into an efficient and modern police force, as well as being granted the title of Duke.

As this book opens, Vimes is anxiously waiting for the birth of his first child and chaffing against the irksome ceremonial responsibilities of his elevated position, so when news comes in that a psychopathic murderer called Carcer (already responsible for the deaths of several watchmen) has been cornered on the roof of the Unseen University. Vimes nobly takes personal responsibility for arresting the villain and through a combination of a sudden thunderstorm and high energy magic is somehow transported back in time.

He quickly realises that he is in the Ankh-Morpork of some thirty years previously, shortly after his younger self had joined the Night Watch, and also that he still has a killer to catch. He assumes the identity of John Keel - the man who mentored him as a young watchman - and who has now been murdered. Time is slowly unravelling and bad things are about to happen.

The theme of this book is revolution, with a mad ruler in charge, a secret police force kidnapping people off the streets and a populace about to take to the barricades. What will happen if the events that Vimes remembers from his past don't play out in the same way?

This is an excellent read, with many enjoyable details about everyday police work in a quasi-medieval city. It's also noticeably darker than some of the other Discworld books with fewer groansome puns and outright comedy characters, and the themes of resistance to tyranny being as depressingly relevant as ever.