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
The first two books written by Ruth Plumly Thompson, "The Royal Book of Oz" (published in 1921) and "Kabumpo in Oz" (published 1922), entered the public domain 75 years later (1997 and 1998, respectively) pursuant to the Copyright Act of 1976.
The next twelve books ("The Cowardly Lion of Oz" through "Speedy in Oz", published between 1923 and 1934) were still copyrighted when the 1998 Sonny Bono Act extended the copyright term a further twenty years, meaning works would now be copyrighted for 95 years. These twelve books of Thompson's are therefore still copyrighted, and will not begin entering the public domain until 2019. By 2030, all of them will be in the public domain.