The Hidden Valley of Oz (1951) is the thirty-ninth of the Oz books created by L. Frank Baum and the only one written by Rachel R. Cosgrove. It was illustrated by Dirk Gringhuis.
Jam (Jonathan Andrew Manley), a boy from Ohio, builds a Collapsible Kite and attaches it to a crate and sets off to Oz with his two guinea pigs, Pinny and Gig, and a lab rat named Percy. Once in Oz, Jam realizes his pets can talk. He lands in the Hidden Valley, and becomes a prisoner, but they escape and set out on adventures with the Tin Woodman.
Her second, The Wicked Witch of Oz (1954) was denied publication on the grounds that the Oz books were not selling. The book was published by The International Wizard of Oz Club in 1993. She did have a tendency to dismiss adult Oz fans and insist that Oz books are "for kids!"
She expresses this view in the documentary, Oz: The American Fairyland.