Equal Rites by Terry Pratchett

It is known as the Discworld. It is a flat planet, supported on the backs of four elephants, who in turn stand on the back of the great turtle A’Tuin as it swims majestically through space. And it is quite possibly the funniest place in all of creation. The last thing the wizard Drum Billet did before Death laid a bony hand on his shoulder, was to pass on his staff of power to the eighth son of an eighth son. Unfortunately for his colleagues in the chauvinistic (not to say misogynistic) world of magic, he failed to check on the new-born baby’s sex.