The Pirates in Oz by Ruth Thompson - 1931

Short summary of Pirates of Oz story:

Peter returns to Oz for a third time, this time with pirates on the Nonestic Ocean (which surrounds the continent Oz is on). Meanwhile, Old Ruggedo, the Gnome King, is back. He had been hit with a Silence Stone at the end of The Gnome King of Oz, and decides to answer an advertisement for king of the Land of Menankypoo. He also encounters Pigasus the flying pig and Captain Samuel Salt and his band of pirates.