In last Oz book, The Emerald City of Oz, magic was used to isolate Oz from all outside worlds. Baum did this to end the Oz series, but was forced to restart the series with this book due to financial hardships. In the prologue, he explains how he managed to get another story about Oz, even though it is isolated from all other worlds. He explains that a child suggested he make contact with Oz with a wireless telegraph. Glinda, using her book that records everything that happens, is able to know that someone is using a telegraph to contact Oz, so she erects a telegraph tower and has the Shaggy Man, who knows how to make a telegraph reply, tell the story contained in this book to Baum.
Warning: Wikipedia contains spoilers.
Ojo the Unlucky is a Munchkin boy who, devoted to life with his uncle Unk Nunkie in the wilderness but on the verge of starvation, goes to see a neighboring "magician" and old friend of Unk's, Dr. Pipt. While there they see a demonstration of the Pipt-made Powder of Life, which animates any object it touches. Unk Nunkie and Dr. Pipt's wife are also the sufferers of the consequences of another of the Doctor's inventions, the Liquid of Petrification, which turns them into solid marble statues.
The remainder of this books is Ojo's quest through Oz to retrieve the five components of an antidote to the Liquid: a six-leaved clover found only in the Emerald City, three hairs from the tip of a Woozy's tail, a gill (a quarter of a pint) of water from a dark well, a drop of oil from a live man's body, and the left wing of a yellow butterfly. With the help of the patchwork girl Scraps, the Glass Cat (another of Dr. Pipt's creations), the Woozy, Dorothy, the Shaggy Man, the Scarecrow, and the Tin Woodman, gathers all of these supplies but the left wing -- the Tin Woodman won't allow any living thing to be killed, even to save another's life.
The party returns to the Emerald City, where the Wizard of Oz (one of the few allowed to lawfully practice magic in Oz) restores Unc Nunkie and Dr. Pipt's wife. The story is also a growth process for Ojo; he learns that luck is not a matter of who you are or what you have, but what you do; he is renamed "Ojo the Lucky," and so he appears in the following Oz books.