Night Watch by Terry Pratchett

The twenty-fifth of May is an important, sombre day in Ankh-Morpork - the anniversary of one of the city's bloodiest rebellions.

But crime stops for nothing, as Commander Sam Vimes of the City Watch knows. When a notorious serial killer emerges from hiding, the chase leads the Watch to the roofs of Unseen University where a magical storm is brewing. It's a case of wrong place, very wrong time.

For Vimes finds himself back in his own rough, tough past with only a killer for company and a city on the brink of revolution to contend with. But he must survive, because he has a job to do: track down the murderer and change the outcome of the rebellion.

And get back to the future before his wife gives birth, of course.

All in a day's work ...