Barbie: The Clue in the Castle Wall (Barbie Mystery Files, #6)

(From the back cover)

A royal mystery...

Barbie and Christie are on their way to England! The two girls can't believe their luck. Barbie is going to interview a real-life princess and Christie is going to take pictures of her crown. But wherever Barbie goes, a mystery is sure to follow. As soon as the two friends reach the castle, the crown mysteriously disappears! Can Barbie and Christie track down the crown...and catch the thief in the act?