The Marvelous Land of Oz is a watershed book in a lot of ways. To the best of my knowledge, Baum hadn't planned on making a series when he wrote the Wizard of Oz. With the Land of Oz, he was at least showing some interest in creating an ongoing series, although I personally don't think the idea is solidified until the third book. I'm sure there are scholars who will prove me wrong.
OK, I'm going to give an overview of the plot of the book. Which means, spoilers galore. The book is free and you can download it from Project Gutenberg so you can do that right now and read it if you want, before I go and spoil it all.
Now that I've dutifully warn you, let me talk about the plot.
The Land of Oz is the only book in the series that doesn't have any characters from the outside world in it. As a side note, that pretty much always means someone from the United States.
In the beginning, our hero, a boy named Tip, is pretty much the slave of the witch called Mombi. It's a pretty low level of slavery, more like an abusive or really strict foster parent. However, when Mombi brings a pumpkin headed manikin to life, she plans on turning Tip into a marble statue since she doesn't need him anymore.
In the beginning, Tip and Jack Pumpkinhead, along with a Sawhorse that they bring to life who serves Jack's ride, are just trying to escape away from Mombi. However, they discover a Revolutionary Army led by General Jinjur who is planning on taking over the land of Oz.
Jinjur makes pretty good progress in conquering the country, particularly when you consider that her basic strategy is to have an army of women who the men won't fight back against. She also gets Mombi as an ally, agreeing to help get Tip. Mombi proves to have some pretty powerful magical chops.
Tip picks up some allies of his own, our old pals the Scarecrow and the Tin Woodman, along with the Woggle-Bug and the flying Gump. Tip isn't just trying to escape Mombi. He now needs to find the lost princess of Oz, Ozma, who is also the rightful ruler of Oz.
Ultimately, Tip is able to get the help of Glenda the Good, who is more powerful than Mombi and who has an army of girls of her own, only they actually are willing to fight. The balance of power shifts and our heroes win the civil war.
The final twist is that Mombi had hidden away Ozma by transforming her into a boy. Yes, Tip has been looking for himself the whole time. One transgender transformation later, Tip has become the fairy princess Ozma and peace at last reigns over the land of Oz.
Seriously, this is probably one of the most dynamic books in the entire series. Oz has a major civil war, the protagonist has a far more extreme revelation than there's no place like home, and the government is completely replaced.
That said, the summation makes the book seem a lot more dramatic than it really is. There's a lot of silly stuff and the entire civil war is tongue-in-cheek social commentary as opposed to a violent confrontation.
As I understand it, one of the driving forces behind Baum writing the book was a success of the stage play of the Wizard of Oz. Said play had a lot more slapstick vaudeville elements then the original book. The Land of Oz definitely has more slapstick bits then really any other book in the series.
As it turns out, since they couldn't get the actors who played the Scarecrow in the Tin Woodman in the play, since they were busy making money hand over fist with the original play, The eventual play got retooled to much more heavily feature the Woggle-Bug, who is possibly the most forgettable and disliked reoccurring character in the entire series. It was a crashing flop. Probably for the best, since the books after the Land of Oz were definitely written as books, as opposed to potential scripts.
As quirky as the Land of Oz is, it does introduce Ozma, who is the second most important character in the series after Dorothy. It also gives us Jack Pumpkinhead and the sawhorse, two more characters who are regular members of the Oz posse in the books to come.
After the Land of Oz, the books settle down to a more consistent tone. And Oz starts getting described as the happiest fairyland in all the world. The Wonderful Wizard of Oz got everything started. The Marvelous Land of Oz, though, so the book that you read to find out what else is out there.