The Land of Oz was heavily influenced by the success of the stage version of the Wizard of Oz. Among other things, the stage play downplayed the role of Dorothy and the Cowardly Lion so neither of them showed up in the second book. The Land of Oz also had a lot of slapstick, which doesn't jibe well with the fairytale atmosphere of Oz.
In other words, I think that Baum forgot his real audience, children. Apparently, quite a few children wrote to him, asking whatever happened to Dorothy and the lion. More importantly, the second stage play was a serious flop.
While the Oz books have a huge and imaginative cast, Dorothy is the most important part of the picture. She is our eyes and ears into the fantastic world. And she also teaches the valuable lesson that a regular little girl can make a big difference, even when she has companions who are so magical.
The book begins with Dorothy as a castaway along with a yellow hen named Billina after they get washed off of the ship going to Australia. (In some ways, a farm girl from Kansas going to Australia for a vacation may be one of the most strange elements of the book.)
Dorothy puts together that she is in a fairy land after Billina gains the power to talk. They have an encounter with creatures called the wheelers, who have wheels instead of hands and feet, and discover the clockwork man Tik-Tok, who becomes their staunch ally.
After Dorothy becomes the captive of Princess Langwidere, who has the disturbing trait of interchangeable heads, Ozma and her entourage arrive, using a magical carpet to cross the deadly desert that surrounds Oz. Amusingly enough, it is not a flying carpet but a red carpet that rolls and unrolls so you can always walk on it.
After they free Dorothy, Ozma and company explained that they are there to save the royal family of Ev, who are the captives of the Nome King.
And here is where the story really goes into high gear. The Nome King transformed his captives into tchotchke and intends to do the same for the Oz crew. Fortunately, Billina overhears him gloating and learns the how he color coded the tchotchkes so she knows how to free everyone.
Of course the Nome King is an evil bastard and refuses to release everyone but Dorothy gains his magic belt. That belt is the most powerful dingus in all the books. Seriously, you do not mess with Dorothy. Cue the triumphant escape.
Everyone is able to happily go home and Ozma also now has the magical belt so she can freely transport Dorothy to and from Oz.
As I said before, this is one of the most tightly plotted books in the Oz canon. Most of the books have at least some element of being a road movie, where our heroes pass through some kind of strange village or land that has nothing to do with the overall plot. In Ozma of Oz, virtually every scene drives the plot forward.
The tone of the book also shifts solidly into what I'm going to call Baum's American Fairy Tale voice. Baum manages to convey ridiculously imaginative settings and characters in a very grounded voice. Baum has his children react realistically to their situations or at least the way that children would like to think they would react. (I'm pretty sure I'd have run screaming from a waking, talking scarecrow when I was a kid... or right now, really)
Ozma of Oz introduced several characters who continue to be important in the series. Perhaps the most important to Tik-Tok, who may be one of the earliest robots and clockwork men in modern fiction. He is unique in all of the strange crowd of Oz by distinctly NOT being alive. Despite that, he goes on to be a very steady part of the Oz posse.
Another very important character who gets added is the Nome King, known as both Roquat and Roggedo. While the Wicked Witch of the West is THE Oz villain, thanks to Margaret Hamilton's brilliant portrayal in the 1939 movie, the Nome King is the big bad of the books. He goes on to be a thorn in Oz's side in three more books and even gets an appearance in one of Baum's rare short stories.l
It has to be admitted that picking fights with the kingdom of Oz is a bad idea. Ozma of Oz sees the Nome King at the height of his power. As the books go, he just falls farther and farther from grace but he remains a determined villain.
While not actually a character per se, the Nome King's magic belt. While most of the magic in Baum's books is very specific in what in can do, the magic belt can do whatever the plot needs it to. Most importantly, it gave an automatic way for to Dorothy to come and go from Oz.
While Ozma of Oz is Billina the Yellow Chicken's only major role, she also makes regular appearances throughout the series and has the virtue of speaking common sense more often than a lot of folks.
The book also introduces what I have to say feels like the most underused character in the Oz posse, the Hungry Tiger. Best friend of the Cowardly Lion and staunch protector of Ozma, the Hungry Tiger craves fat babies to eat but his conscious will never let him. Honestly, it feels like his character development took place in a missing book because he feels like he should have a backstory like the lion or the Tin Woodman or the Scarecrow.
Ozma of Oz is a very good book. It brings back Dorothy to her place as eyes and ears of Oz and builds a foundation of tone and situations and characters that served Baum for many books to come.