The World Above by Cameron Dokey

Gen and her twin brother, Jack, were raised with their mother's tales of life in the World Above. Gen is skeptical, but adventureous Jack believes the stories--and trades the family cow for magical beans. Their mother rejoices, knowing they can finally return to their royal home.

When Jack plants the beans and climbs the enchanted stalk, he is captured by the tyrant who now rules the land. Gen sets off to rescue her borther, but danger awaits her in the World Above. For finding Jack may mean losing her heart....

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