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..
Little Wizard Stories of Oz
None of the new series was as successful as the previous Baum and Van Dyne series; the Oz books and "Aunt Jane's Nieces". Both the "Flying Girl" and "Daring Twins" series ended with their second volumes, "The Flying Girl and Her Chum" and "Phoebe Daring", both published in 1912. Disappointing sales through 1911 and 1912 convinced Baum and Reilly & Britton that a return to Oz was needed. Baum wrote "The Patchwork Girl of Oz" for a 1913 release, and in the same year his publisher issued the six "Little Wizard" stories in individual booklets at a cost of $0.15 each. The goal was to reach the youngest beginning readers, and create in them an interest in the larger Oz canon, as part of a "promotion of L. Frank Baum and all of his books."