The Hidden Valley of Oz, by Rachel R. Cosgrove

Delightful. Thumbs up. This is one of the few non-Baum Oz books that's worth reading.

A boy named Jam from Ohio finds his way to Oz via a large kite. He lands in Gillikin Country. The supporting characters are memorable: his guinea pigs named Pinny and Gig, and his pet rat Pervical, aka Percy the Personality Kid. Percy is a lot of fun.

They end up in in a valley where the citizens toil as slaves of the giant Terp the Terrible. Terp is fifty feet tall, and he forces the citizens to tend the grapes and make jam, which he eats for breakfast on muffins from his magic muffin tree.

Percy helps Jam escape from the valley, and steals some of Terp's magic muffins. It turns out the muffins are what make Terp a giant; when Percy eats one, he becomes human-sized. But the magic only lasts a few days.

Jam and Percy meet Dorothy, the Scarecrow, the Tin Woodman, the Cowardly Lion, and the Hungry Tiger, and together they have many adventures, finally returning to Terp's valley and chopping down the magic muffin tree. Terp shrinks back to normal size, and the citizens are free. Ozma sends Jam home and the Wizard makes Percy's large size permanent, so it's a happy ending all around.