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The Hidden Valley of Oz is the thirty-ninth installment in the Oz book series. It is the first Oz novel written by Rachel Cosgrove.


Jonathan Andrew Manley, nicknamed Jam, is a boy from Ohio, the son of a biologist. As the story starts he is building a "collapsible kite" from plans in a popular magazine. Instead of cutting down his wooden frame to match the plans, Jam scales up the kite to fit his wood, giving him an extra-large result. He thinks it might support him; he attaches a shipping crate and brings along three of his father's lab animals, two guinea pigs and a white rat. A strong gust of wind lifts kite, crate, and passengers into the sky. Jam is on his way to the Land of Oz.

The kite lands in a remote valley in the Gillikin Country the next day. Jam learns that his animals can now talk; the guinea pigs call themselves Pinny and Gig, while the rat claims to be Percy, "the personality kid." This valley is dominated by a ruthless giant, 50 feet tall, called Terp the Terrible. He enslaves the common people and forces them to work in his vinyards and his jam-making factory. Terp captures Jam and is impressed with his name. Terp threatens to spread the boy on muffins and eat him for breakfast.

With Percy's help, Jam escapes confinement and sets off on an Oz adventure that leads him to confront hostile centaurs called Equinots and threatening locals in Bookville and Icetown. He has powerful allies, though, in Dorothy, the Scarecrow, the Tin Woodman, the Cowardly Lion, and the Hungry Tiger. Together, this crew meet a new feline friend, the Leopard with the Changeable Spots.

Percy grows to ten times his normal size by eating a magic muffin, which reveals Terp's secret and the way to defeat him. The Tin Man chops down the giant's magic muffin tree; Terp is imprisoned until the magic expires and he shrinks to normal size. The victors are welcomed to an Emerald City triumph. Ozma sends Jam back home with her Magic Belt, though Percy stays in Oz, and gets the Wizard to enchant him into his larger size permanently.


In her original draft, Cosgrove sent Jam to Oz in a rocket. Reilly & Lee required a change, since rocket transport had already been used in The Yellow Knight of Oz (1930). Cosgrove's original beginning was published posthumously in Oz-story Magazine No. 6 (2000).

Cosgrove also composed an article detailing how she worked with the personnel at Reilly & Lee to complete the book and prepare it for publication. Her article was printed in The Baum Bugle in 1991, and was also included as an Afterword in the edition of Hidden Valley released by The International Wizard of Oz Club that year.

In both the text and illustrations of Hidden Valley, the giant Terp seems too big for his castle. L. Frank Baum manages giantism better in his 1913 tale "Little Dorothy and Toto," in which the giant Crinklink can change his size from enormous to tiny.

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The Final "Famous Forty" Books
John R. Neill 34. The Wonder City of Oz 35. The Scalawagons of Oz 36. Lucky Bucky in Oz
Jack Snow 37. The Magical Mimics in Oz 38. The Shaggy Man of Oz
Rachel Cosgrove 39. The Hidden Valley of Oz
Eloise Jarvis McGraw and Lauren Lynn McGraw 40. Merry Go Round in Oz