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..
The Royal Book of Oz
The Royal Book of Oz (1921) is the fifteenth in the series of Oz books, and the first, by Ruth Plumly Thompson, to be written after L. Frank Baum's death. Although Baum was credited as the author, it was written entirely by Thompson. Beginning in the 1980s, some editions have correctly credited Thompson, although the cover of the 2001 edition by Dover Publications credits only Baum. The original introduction claimed that the book was based on notes by Baum, but this has been disproved. Baum's surviving notes, known as "An Oz Book" are known from four typewritten pages found at his publisher's, but their authenticity as Baum's work has been disputed. Even if genuine, they bear no resemblance to Thompson's book.