Copyright status of The Wizard of Oz and related works in the United States
"The Wonderful Wizard of Oz" was published in 1900, and was the first to enter the public domain in "1956". This allowed Reilly & Lee, the publisher of all the other Oz books (as well as Baum's other books) to issue their own edition of "The Wizard of Oz", which had previously been published by George M. Hill Company and later by the Bobbs Merill Company after Hill went bankrupt..
Copyright status of The Wizard of Oz and related works in the United States
All of Baum's subsequent Oz books, published between 1907 and 1920, had their copyright extended by subsequent laws. Between 1962 and 1974, Congress passed nine copyright acts extending the copyright one or two years each time (these acts also introduced the rule that rounds the copyright term up to the end of that calendar year). This ensured that all works published after September 1906 (and renewed as necessary) stayed copyrighted until the Copyright Act of 1976 took effect. The 1976 Act extended the renewal to 47 years, meaning works could stay copyrighted for 75 years after publication. Thus, the rest of Baum's Oz books did not enter public domain until the 75-year terms was up. "Ozma of Oz" (published in 1907) was the first to enter the public domain in 1983, and the subsequent books followed, with "Glinda of Oz" (published in 1920) finally entering the public domain in 1996.