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..
Dorothy and the Wizard in Oz
Baum, having resigned himself to writing a series of Oz books, set up elements of this book in the prior "Ozma of Oz" (1907). He was not entirely pleased with this, as the introduction to "Dorothy and the Wizard of Oz" opens with the protest that he knows many tales of many lands, and hoped that children would permit him to tell them those tales.