A Fiery Friendship is the first novel in a prequel Oz book series created by Gabriel Gale, written by Lisa Fiedler, and published by Simon & Schuster. It was released on May 16, 2017. It is part of a larger project called Ages of Oz.
(from the book's website)
On her Declaration Day, a day meant for celebration and happiness, Glinda’s peaceful life in Oz is shattered when her mother is imprisoned for practicing forbidden Magic. As she is ripped from her home by a fearsome bounty hunter sent by Aphidina, the Witch of the South, Glinda soon uncovers a startling truth: the Oz she’s always know is not good and right—it’s a world governed by the wickedest of the wicked, overrun with tyranny, corruption, and dark power. And Glinda’s mother? She is actually a high-ranking member of a secret society whose mission is to overthrow the four Wicked Witches and set the stage for the return of the rightful ruler of Oz.
With the help of a feisty, purple-haired girl named Locasta, Glinda sets across the unforgiving landscape to rescue her mother. They are soon joined by a boy named Ben, a revolutionary New Yorker brought to Oz through a blizzard, and a mysterious girl named Shade. Armed with their individual gifts, these unlikely heroes mount an epic attack on Aphidina to free Glinda’s mother…and save the future of Oz from the Wickeds before it’s too late.