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.