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.

Equal Rites, by Terry Pratchett (1987, UK)

An old wizard is about to die and goes to Bad Ass to leave the magic staff to the 8th son of an 8th son...who turns out to be a daughter instead. No woman wizard can be. On the other hand, no witch with magic wizard staff can be. Trained by Granny Weatherwax, Eskarina is going to be a very special person. Very ironic and hilarious book.