The Wizard of Oz (1902 musical)
The Wizard of Oz was a 1902 musical extravaganza based on "The Wonderful Wizard of Oz" by L. Frank Baum, which was originally published in 1900. Much of the original music was by Paul Tietjens and has been mostly forgotten, although it was still well-remembered and in discussion at MGM in 1939 when the classic film version of the story was made. Although Baum is the credited bookwriter, Glen MacDonough was hired on as jokewriter after Baum had finished the script.
Copyright status of The Wizard of Oz and related works in the United States
Alexander Volkov's six Oz books were published in the Soviet Union, so they are subject to Russian copyright law. The first book – "The Wizard of the Emerald City" – was published in 1939, when "The Wizard of Oz" was still copyrighted. "The Wizard of the Emerald City" is an adaptation of "The Wizard of Oz", borrowing liberally from the original book without even crediting Baum.
The subsequent four books were published between 1963 and 1970, and the last one – "The Secret of the Deserted Castle" – was published in 1980. At the time of publication, "The Wizard of Oz" and "The Land of Oz" were in the public domain, but the other Oz books were still copyrighted. However, Volkov's books are mostly original sequels to "The Wizard of the Emerald City", so whether they are infringing or not is difficult to say. Barely any of the elements from later Oz books are featured in Volkov's works, so most likely they were not infringing.