Jack Pumpkinhead of Oz is the twenty-third volume in the Land of Oz book series, and the ninth written by Ruth Plumly Thompson, published in 1929.
Peter Brown, a repeat visitor to Oz by this point, mopes in his attic, unable to play baseball due to the rain. In the attic he finds the sacks that were full of gold when he brought them back from his first Oz adventure two years earlier (in The Gnome King of Oz); and one of those sacks contains an odd gold coin. Toying with the coin and thinking of Oz, Peter wishes himself back there and suddenly finds himself in Jack Pumpkinhead's front yard.
The sensible thing for Peter to do is to head for the Emerald City; and Jack is ready to act as his guide. Guiding is not Jack's strong suit; the two quickly get lost in the Quadling Country, where they blunder into Chimneyville and Scare City. By chance, Peter finds that his empty sack will consume objects and creatures that are scooped into its open mouth. Jack and Peter also happen to obtain the magic dinner bell of Jinnicky the Red Jinn, which supplies Peter with needed provisions.
The two travelers adopt a third member for their party when they meet the doggerel-spouting Snif the Iffin (he's a griffin who has lost his "gr-"). The three then encounter the unfortunate Belfaygor, the Baron of Bourne. He has been accidentally cursed with a rapidly-growing beard that he must constantly snip away. Even worse, his fiancee, the princess Shirley Sunshine, has been kidnapped by the local villain, Mogodore the Mighty, the Baron of Baffleburg. To impress her and gain her fancy, Mogodore decides he will conquer the Emerald City and make himself king.
- And Now You Must Marry Me: What Mogodore does when he captures Shirley Sunshine; and later, when he sees how pretty Ozma is, he changes his mind and decides he will marry Ozma.
- Bag of Holding: Peter finds one in Scare City, and uses it to capture the spooks scaring them.
- Cranium Chase: This happens to Jack understandably often; losing his head in Mogodore's castle however enables him to evesdrop and gain important information about the Forbidden Flagon.
- Damsel in Distress: Shirley Sunshine is a textbook example.
- Easily Conquered World: Oz has always been this (protected mainly by the Deadly Desert), but it reaches new levels of ineptitude in this book. Mogodore easily conquers the Emerald City while every single person of importance in the palace is playing Blind Man's Bluff, and is blindfolded.
- Exact Words: The Forbidden Flagon claims it will bring disaster upon the head of whoever uses it; Jack takes his head off first before using it to defeat Mogodore.
- Filler: The first few chapters, particularly their visit to Chimneyville.
- Flat Character: The main complaint many readers have about Peter, who is little more than an Audience Surrogate.
- Made a Slave: Two of Mogodore's underlings suffer this fate after being teleported to the Red Jinn (a recurring protagonist in Thompson's books) by his slave.
- Our Genies Are Different: Jinnicky the Red Jinn is a small magical man who keeps his body encased in a jar and commands an army of slaves (yet he is depicted as benevolent). Peter and Jack find Jinnicky's bell which magically summons his black slave who provides them with meals. He is one of the main reasons Thompson's Oz books have aged far less gracefully than Baum's.
- Pumpkin Person: Jack, of course.
- Rapid Hair Growth: Belfaygor's endlessly growing beard; which comes in handy in odd situations such as when the party needs to cross a deep chasm.
- Save the Princess: This becomes the party's goal after they meet Belfaygor. Later they need to save Ozma and her friends.
- Simpleminded Wisdom: Jack's characteristic wisdom shines through at some points, despite mostly being a simpleton.