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 Sea Fairies
Baum had decided to end the Oz series with "The Emerald City of Oz" in 1910, after six installments over the first decade of the twentieth century. "The Sea Fairies" was intended to be the first in a new series of fantasy novels, which Baum and Reilly & Britton continued the next year with "Sky Island." Unfortunately for author and publisher, the two volumes of the new projected series did not meet with the same success as the Oz books previously had. The first edition of "The Sea Fairies" sold 12,400 copies in its initial year on the market, where "The Emerald City of Oz" had sold 20,000. Even when Baum's books experienced a major resurgence in interest and sales in 1918, "The Sea Fairies" sold only 611 copies that year while the Oz books and even Baum's non-Oz works were selling thousands of copies.