Little confession. I got The Seven Blue Mountains of Oz trilogy by swapping for them. However, when I was making the deal, they asked if I had Thorns and Private Files in Oz. I said, "No," so they threw it in. The illustrator of the book at hand, Marcus Mebes, confirmed that it should be read as The Seven Blue Mountains of Oz Book 2.5. So, I did.
The book is by Melody Grandy and Chris Dulabone. Writing styles of both are clearly evidenced in the text. According to the introduction, Marcus' pictures of some friends as a prince and princess bore a big resemblance to Jo Files and Ozga from Tik-Tok of Oz, inspiring the story. (You never know when you might influence someone to write an Oz story!)
You might think it's called Thorns and Private Files in Oz because Files is in it, right? Well, he is. But it turns out thorns and files that are private actually come into play in the story. How, I won't tell you exactly why.
Files and Ozga are living happily ever after in Oogaboo, reading and tending the book trees. However, when some of the stories on their trees prove to be Oz stories about the future, they and Hank the Mule (who's visiting) have to keep them away from Queen Ann. They manage to make the tree grow fiction, but not before Ann steals a few books. After they trick her with a copy of Dorothy Returns to Oz, they hide out in the forest, where they find a deserted castle full of overgrown rose bushes and thorns. However, Ozga disappears and Files and Hank must trust a gander and a curious old hermit woman to get her back.
The story feels incomplete. As it seems the plotline will resume in the final volume of The Seven Blue Mountains of Oz (the final illustration, by Melody Grandy, shows Files, Ozga and Hank with Dinny and Zim, which was later reused on the last book of the trilogy), it was a very odd piece on its own. At least it says "To Be Continued."
I guess I'll see if The Seven Blue Mountains of Oz: Zim Greenleaf of Oz closes this story suitably.