Night Watch — Terry Pratchett

One moment, Sir Sam Vimes is in his old patrolman form, chasing a sweet-talking psychopath across the rooftops of Ankh-Morpork. The next, he's lying naked in the street, having been sent back thirty years courtesy of a group of time-manipulating monks who won't leave well enough alone. This Discworld is a darker place that Vimes remembers too well, three decades before his title, for-tune, beloved wife, and impending first child. Worse still, the murderer he's pursuing has been transported back also. Worst of all, it's the eve of a fabled street rebellion that needlessly destroyed more than a few good (and not so good) men. Sam Vimes knows his duty, and by changing history he might just save some worthwhile necks--though it could cost him his own personal future. Plus there's a chance to steer a novice watchman straight and teach him a valuable thing or three about policing, an impressionable young copper named Sam Vimes.