Night Watch, by Terry Pratchett (2002, UK)

Discworld, Ank Morpork. Commander Vimes is about to arrest Carcer in the Unseen University magic library... when they are brought back to 30 years in the past. Everything has still to happen and, although Vimes only wants to get back, they both can't help but living there ...meeting the young themselves perhaps and messing up everything. But history always finds a way. Fascinating.