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
Snow published two canonical Oz books: "The Magical Mimics in Oz" in 1946, and "The Shaggy Man of Oz" in 1949. The copyright was never renewed on either of these, so they entered the public domain in 1974 and 1977, respectively. They are the last two Oz books that entered the public domain.