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..
List of Oz books
The Oz books form a book series that begins with "The Wonderful Wizard of Oz" (1900) and relate the fictional history of the Land of Oz. Oz was created by author L. Frank Baum, who went on to write fourteen full-length Oz books. All of the Baum written books are in the public domain in the United States. Even while he was alive, Baum was styled as "the Royal Historian of Oz" to emphasize the concept that Oz is an actual place. The illusion created was that characters such as Dorothy and Princess Ozma related their adventures in Oz to Baum themselves, by means of wireless telegraph.