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..
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.