How the Wizard Came to Oz was a 1991 prequel to The Wonderful Wizard of Oz by Donald Abbott. The book tells the story of the Wizard of Oz's arrival in Oz, as the title implies, and his subsequent first meeting with the Wicked Witch of the West. It also depicts several other events that lead up to Dorothy's arrival.
High winds draw Oscar Diggs' hot air balloon into Oz, where he encounters the Wicked Witch of the West in Winkie Country. When she threatens to use her magic on him, he scares her off with his ventriloquism. He soon becomes the new lord of the Winkies, while the Wicked Witch retreats to find a way to defeat her new nemesis. On advice from her sister, the Wicked Witch of the East, she steals the Golden Cap from the sorceress Gaylette and commands the Winged Monkeys to chase Oscar away.
He is saved from being pursued further by Glinda and lands in the future site of the Emerald City. He befriends the newly named Ozites and deals with a Kalidah Prince before going on to create the Emerald Spectacles and puppets to further the illusion that he can change form. The book ends with Oscar defending the Emerald City from an attack by both Witches with more secret help from Glinda.
The author writes in a close imitation of Baum's writing, and the illustrations look practically identical to W. W. Denslow's as they appear in The Wonderful Wizard of Oz.