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
Four of the "Little Wizard Stories" were re-issued in 1932 in a new form, as "The Little Oz Books with Jig Saw Oz Puzzles". A year or two later the four tales were released again, as part of a promotion for a "Wizard of Oz" radio program (sponsored by Jell-O). Rand McNally published the six stories in three booklets in 1939.