Harry Baum pursued a career in various businesses though his adult life, including hotel management. He also wrote: he collaborated with Olive Beaupre Miller on a quartet of history books for children, The Story of Mankind: A Picturesque Tale of Progress (1929 and after), also called My Book of History. He may have helped his father with the third in the series of Baum's Marie Lousie books, Marie Louise Solves a Mystery (1917).
H. N. Baum also wrote about his father. His article "How My Father Wrote the Oz Books" appeared in the December 1962 issue of the American Book Collector devoted to Baum. His "My Father was 'the Wizard of Oz': Memories and Anecdotes of a Famous Father" was published in The Baum Bugle, Autumn 1985.
He married three times in his life. At the time of his death in 1967, Harry Baum was the last surviving son of L. Frank Baum.