I bought all these books in paperback in the 1970s when they were first released by DelRay, and I still have those. They are still so much fun to read and because Mr. Baum was writing long before authors were urged to write specifically for certain age groups, the stories are not necessarily geared for children. He wrote for anyone and didn't restrict his vocabulary, so it doesn't feel like you're reading a "children's" book. The stories are incredibly inventive, the plots to some of the later ones having been suggested by his fans. All the books have an overarching theme of tolerance (very timely today), honesty, fairness, etc. In fact, tolerance for individuality and alternate lifestyles is such a prevalent theme, that I'm surprised Baum is not condemned by right-wing conservatives. These are WONDERFUL books that maybe I'll re-read again when I'm 80.
UPDATE: I just finished the entire thing and have to say my favorite ones are "The Emerald City of Oz" (wonderfully involved and complex plot with a touch of realism), and "Rinkitink in Oz." "Rinkitink" doesn't even take place in Oz and has no Oz characters until the very end. It shows that Baum could write more than "Oz stories" with the same characters. In fact, I thought it was a great story and that having Dorothy come in at the end sort of ruined it.
The first Ruth Plumly book left much to be desired, although her second one, the last in this series, was a vast improvement. Still, her sense of fantasy was not grounded in the same reality that Baum's was; she gets a little...weird. Like the "runaway country" character, an enormous piece of land, with legs, and lakes for eyes, that runs around abducting people to colonize it. She also lacks Baum's sense of humor and sense of irony. You won't be missing anything if you just read the first 14 full Baum books. Enjoy!