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..
Land of Oz
Originally, Baum did not intend for "The Wonderful Wizard of Oz" to have any sequels, but it achieved a greater popularity than any of the other fairylands he created, including the land of Merryland in Baum's children's novel "Dot and Tot in Merryland", written a year later. Due to Oz's worldwide success, Baum decided to return to it four years after "The Wonderful Wizard of Oz" was published. For the next two decades, he described and expanded upon the land in the Oz Books, a series which introduced many fictional characters and creatures. Baum intended to end the series with the sixth Oz book "The Emerald City of Oz" (1910), in which Oz is forever sealed off and made invisible to the outside world, but this did not sit well with fans, and he quickly abandoned the idea, writing eight more successful Oz books, and even naming himself the "Royal Historian of Oz."