Equal Rites (Discworld, Bk 3)

Plodding through the eternal void is the great turtle A'Tuin. On his back (or hers - the question is unresolved) stand four elephants.

And supported on the elephants' shoulders is. . . Discworld, planet of magic and misadventure!

Predicting his own death, the wizard Drum Billet sets out to pass on his power and his staff to his predicted successor, the eighth son of an eighth son. But there is a problem. The eighth son turns out to be a daughter, and women aren't supposed to be wizards.("Where does it say women can't be wizards? It doesn't say it anywhere, it says it everywhere.")

But it's too late: Eskarina inherits the wizard's staff, and with the reluctant help of the witch Granny Weatherwax sets out to learn her new calling.