Who wrote the first authorized Oz sequel after L. Frank Baum's death?

Ruth Plumly Thompson wrote the first authorized Oz sequel published after L. Frank Baum’s death.

That book was The Royal Book of Oz, published in 1921 as the fifteenth Oz book. Baum’s name appeared on early editions, but the text was written entirely by Thompson. Reilly & Lee, Baum’s publisher, had selected her to continue the series as its new “Royal Historian of Oz.”

The book follows the Scarecrow as he becomes troubled by the question of his origins and returns to the cornfield where Dorothy first found him. Dorothy and the Cowardly Lion search for him, leading to a new adventure that expands Oz beyond Baum’s original novels.

The wording can cause confusion because Thompson’s book was initially presented as though Baum had authored or at least substantially prepared it. Later editions and scholarship more clearly credit Thompson. The wrong options were all connected to later Oz publishing: John R. Neill wrote subsequent books, Jack Snow continued the series in the 1940s, and Rachel Cosgrove Payes wrote a much later entry.