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
Thompson did not renew the copyright on her last five canonical books: "The Wishing Horse of Oz", "Captain Salt in Oz", "Handy Mandy in Oz", "The Silver Princess in Oz", and "Ozoplaning with the Wizard of Oz". Thus, these books entered the public domain 28 years after publication, between 1963 and 1967.
Congress amended the copyright law in 1992 so there would subsequently be automatic renewals of copyright. Therefore, the copyright on Thompson's deutero-canonical books "Yankee in Oz" (published in 1972) and "The Enchanted Island of Oz" (published in 1976) will not expire until 2068 and 2072 respectively.