The origin of the lengthy Oz series -- a series that soon showed that demanding the author continue is not always wise -- but since they are at the beginning, the series is still going strong.
You may think you know Wizard from the movie, but rest assured, changes were made. For one thing, in the book this scene happens right after she meets the Scarecrow
"I cannot understand why you should wish to leave this beautiful country and go back to the dry, gray place you call Kansas."
"That is because you have no brains," answered the girl. "No matter how dreary and gray our homes are, we people of flesh and blood would rather live there than in any other country, be it ever so beautiful. There is no place like home."
The Scarecrow sighed.
"Of course I cannot understand it," he said. "If your heads were stuffed with straw, like mine, you would probably all live in beautiful places, and then Kansas would have no people at all. It is fortunate for Kansas that you have brains."
Why, therefore, did Glinda not tell her the secret when she had first arrived? Because she wasn't there. The Good Witch of the North sends her on her way. A way that involves a lot more episodes than made it into the film. And also makes it a lot clearer than the Scarecrow is smart, the Tin Woodman tender-hearted, and the Cowardly Lion brave.
Land takes place wholly in the land of Oz. The only real series problem is that you can see the inconsistencies between this and Wizard. It's about Tip, who's being raised by a witch, Mombi. When his attempt at scaring her with a pumpkin-headed figure only gives her a way to test her Dust of Life, it still means that she decides to turn him to a statue as too much trouble. He takes Jack Pumpkin-Head with him, and steals the Dust of Life, so they end up bringing a saw-horse to life as well as they head to the Emerald City.
The rest involves any army -- two armies -- of girls, Mombi's abilities to make the road bewildering, sunflowers, a Highly Magnified bug, wishing pills, Glinda's refusal to restore the Scarecrow to the throne, an interpreter, and more.