The Emerald City of Oz is the sixth book in L. Frank Baum's Land of Oz book series, published in 1910. It was illustrated by John R. Neill. Baum, tired of writing the Oz series and wanting to move on to other works, intended it to be the last Oz book; instead it is more of a Series Fauxnale, as Baum would be forced to write the seventh Oz book, The Patchwork Girl of Oz, just three years later due to financial hardship.
The Nome King, Roquat the Red, plans to dig a tunnel under the Deadly Desert into the Land of Oz where he will take back his Magic Belt from Princess Ozma, destroy Oz and enslave its people. General Guph then tours the neighboring lands rounding up allies, including the comical Whimsies, the strong and brutal Growleywogs, and the powerfully magical Phanfasms in preparation for the invasion. Princess Ozma eventually discovers the plan using the Magic Picture, but pledges pacifism and resolves not to meet the invasion with force.
Meanwhile, Dorothy Gale's family is in dire financial straits; having to rebuild their home after the tornado in the first book, coupled with years of droughts, have put Uncle Henry in so much debt that their farm is going to be foreclosed on. Dorothy makes a desperate plea to Princess Ozma to allow herself, her aunt and her uncle to live in Oz permanently, to which Ozma readily agrees. Dorothy's aunt and uncle are amazed to learn Oz was real all this time, and while they need some time to adjust to being transported to a magical land, they are all too happy to live in Oz. They then tour the southern Quadling Country of Oz in Ozma's Red Wagon and meet various intriguing people like Miss Cuttenclip, the Fuddles, and the worrisome Flutterbudgets. Dorothy also becomes lost one morning and visits Utensia, Bunbury, and Bunnybury before the Wizard finds her again. However, when they visit the Tin Woodsman, they learn about the impending invasion of Oz, and race back to the Emerald City to meet their fate.
- Actual Pacifist: Ozma, to the degree where she refuses to even organize a resistance against the invaders; granted it is stated that no one in Oz could even stand up to them. Luckily though, she and her friends find a nonviolent solution before this becomes Suicidal Pacifism.
- Amnesiacs are Innocent: This is the trope that saves Oz, as even the malicious Phanfasms are rendered harmless after drinking from the Fountain of Oblivion. The Nome King is turned innocent too by the end of the book, but by Tik-Tok of Oz he regains his cruel streak.
- Chekhov's Gun: The Nomes and a few other unruly tribes of creatures plan to invade Oz, destroy it, and enslave the people. The surprise is initially ruined by Ozma's convenient Magic Picture, allowing her to plan ahead of time. With the Magic Belt Dorothy stole from the Nome king in a previous book, Ozma uses its power to dehydrate the army, whose invasion tunnel is conveniently right next to the fountain containing the Water of Oblivion, which makes anyone who drinks of it forget everything. The first thing the invaders do when they come out of the tunnel is drink the water; war avoided.
- Cooperation Gambit: General Guph knows very well that the "allies" he is collecting intend to backstab the Nomes once Oz is conquered, however he focuses on the task at hand, advising the Nome King to simply use his magic belt to deal with them once he reclaims it from Ozma.
- Fish out of Water: Aunt Em and Uncle Henry are very much fish out of water in Oz, and have some difficulty adjusting to their new lives.
- For the Evulz: General Guph's entire motivation for invading Oz is because he hates happy people and wants to make them all unhappy. This goal, as it turns out, also gives the Phanfasms reason enough to join the invasion.
- Grand Finale: Has all the markings of one, with Dorothy coming to live in Oz and all of Oz's enemies mounting an invasion of the land. Marvel Comics' series of Oz comics ends with this story, complete with a Creator Cameo by L. Frank Baum.
- HeelFace Brainwashing: Essentially what the Fountain of Oblivion does to the invaders of Oz, via erasing their memories. In the Nome King's case it doesn't stick in future books.
- Hidden Elf Village: Glinda and Ozma decide to make Oz an entire "Hidden Elf Country" (minus elves) in response to the invasion, also taking into account that humans have invented airships that could fly across the deadly desert and discover Oz.
- Hurricane of Puns: The kitchen-supplies-based kingdom of Utensia. A colander is the priest, because he's the holiest one there. A corkscrew is a lawyer, because he's accustomed to appearing at the bar; he's a corking good lawyer, but accused of being crooked, and laments that he has no pull at this court. Inadvertently back then, he often screws people over. The knives make sharp remarks. The fork has a tinny voice. It just keeps going.
- Hypochondria: The Flutterbudgets, citizens of Flutterbudget Center, all suffer from this, along with excessive worrying over things that are unlikely to occur. Ozma ordered the Flutterbudgets to be quarantined in one city so that they wouldn't bother the rest of Oz with their excessive worrying.
- I Choose to Stay: It took four adventures, and the inevitability of her family losing their farm, but at long last Dorothy chooses to stay in Oz permanently. And her friends couldn't be happier.
- Idiosyncratic Episode Naming: Each chapter title begins with the word "How", the only Oz book that does this.
- Inconvenient Summons: When Ozma teleports Aunt Em and Uncle Henry to Oz, Em is in the middle of cleaning dishes, while Henry was in the barn tending to animals, only to instantly find themselves in her throne room, to their shock. Later on Henry complains that if he'd known he was going to end up in a royal palace he would have dressed in his Sunday clothes.
- Laser-Guided Amnesia: Oz is saved via the Fountain of Oblivion, a fountain in the Emerald City that erases one's memories when drunk from. Ozma casts a spell in the tunnel the Nomes built creating a lot of dust and making the enemy's throats dry, so that when they burst through the ground the first thing they did was drink from the fountain. Ozma then teleported the childlike monsters away back to their homes.
- Legion of Doom: General Guph manages to form one of these out of villainous tribes, who join either out of greed or because they just want Oz to suffer.
- Level Ate: Bunbury, a town in Oz made entirely of bread and baked goods.
- Motor Mouth: The people of Rigamarole were sent to live in an isolated town due to their inability to stop talking.
- Road Trip Plot: The bulk of the novel is basically two parallel road trips, one of Dorothy and friends touring Oz, and one of General Guph recruiting evil allies.
- Series Fauxnale: Baum's young fans of the 1900's just wouldn't let him stop there. Fortunately it's possible to send telegraphs from Oz to the outside world.
- Surprisingly Realistic Outcome: It turns out that losing their house in the first book to a tornado had some very real and dire consequences for Dorothy and her family; this is the first book in the series that really mentions them. The possibility of Dorothy having to do child labor for their survival is even mentioned; but of course, who wants to do that when they can go to a fantasy world and be a princess?
- Tiny-Headed Behemoth: The Whimsies are one of the groups that conspires with the Nome King to conquer the Emerald City. They are huge, hulking humanoids who have heads that are very small for their bodies. They wear large, garishly painted masks to cover their heads.
- Two Lines, No Waiting: For most of the book the chapters alternate between Dorothy and her friend's adventures in Oz and General Guph's misadventures recruiting allies to invade Oz.
- Wacky Wayside Tribe: Several. Unlike the previous book, which is often criticized as anticlimactic for its reliance on this trope, Dorothy and her family's tour of the wacky wayside tribes of Oz being juxtaposed with Guph's quest to destroy Oz does do something to increase the urgency in the story, showing the reader what's at stake.