Night Watch, by Terry Pratchett.
It's basically about a recurring character in Pratchett's books, Sam Vimes. He is the chief of police and has a pretty good life. Because of a freak magic accident (which is explained well, by the way), he and a really evil criminal are thrown back in time by about 30 years. That just happens to be when Vimes first joined the Watch, when he was still a child. Of course, he accidentally kills the man who taught him anything, but because Time has a funny way of fixing itself, he ends up with a scar and eye-patch just like the man who taught him everything.
Now the city is on the brink of a cival war. The man he replaced was supposed to play a major part in the war, and the man who he was chasing at the beginning plays another major part.
It's actually a pretty amazing book. I love it a lot. I'll post my two favorite quotes:
Sidoh should like this one, it's a Monk of Time explaining the nature of Time to Vimes:
"We used to think that time was like a river, you could row up and down and come back to the same place. Then we found out it acted like a sea, so you could go from side to side as well. Then it turned to be like a ball of water; you could go up and down too. Currently we think it's like . . . oh, lots of spaces, all rolled up. And then there are time jumps and time slips and humans mess it up too, wasting it and gaining it. And then there's quantum of course." The monk sighed. "There's always bloody quantum"
Vimes talking to his younger self about the Secret Police:
"Yeah, all right, but everyone knows they torture people," mumbled Sam
"Do they?" said Vimes. "Then why doesn't anyone do anything about it?"
"'cos they torture people!"
