My sisters and I read several of the Oz books as children, and watched the movie countless times.

"Oz" became one of the first popular Fantasy-genre series, and when Baum passed away, quickly became a shared-world one as well, as Ruth Thompson then other authors took up that official mantle.

As it's passed out of copyright, the whole thing has become a free-for-all.

BTW, there's a great made-for-TV biopic starring John Ritter as L. Frank Baum.