When I was a kid I was very much into the Oz books, all fourteen of them
Imagine my surprise at age 9 when I went with my parents to a mall in New Jersey and saw a slew of new ones!
Unbeknownst to me, Ruth Plumly Thompson had been appointed Royal Historian of Oz after Baum and wrote 19 more in the series.
It was like pulling teeth from my dickhead father to get even two books. He assured me I could get more later and was completely indifferent to why this was such a big deal to me.
Sure enough, they went out of print soon after and became impossible for me, as a child, to find.
Of the 19, 8 were public domain and reprinted as hardcover. The remaining 11 I tracked down as an adult.
Recently I decided to upgrade them to the hardcovers with dustjacket, and now I have all of the so-called Famous Forty (yep there were more after Thompson).