Out Of Their Minds: The Minds Series, Book Three (Hardcover) by Professor Perry Nodelman, Carol Matas

Princess Lenora and Prince Coren are finally getting married at the court of Andilla, Coren's homeland, where everyone lives in their imagination. There's just one problem: The Andillans' powers aren't working, and the wedding can't proceed when the sacred Balance is so out of whack. For the headstrong Lenora, this means a new adventure. She and a reluctant Coren head into the Andillan countryside, where they come across the Skwoes, who have absolutely no imagination whatsoever. Are they responsible for the Andillans' loss of power? Whatever the source, the problems are getting worse: Now even Lenora's ability to make her thoughts real is failing. What is more, Lenora has been having a series of nightmares about the evil Hevak. Something has to be done before all of the people in two kingdoms are completely out of their minds.