This was a really cute mystery in the "Nate the Great" series.
Here, Nate the Great is sent a note from his friend Rosamond, who is vacationing in Scandinavia. She has lost something on a visit to one of the King of Sweden's palaces. In order to recover it, she enlists Nate's help. If he doesn't assist her, she threatens to recruit the King of Sweden to help. Can Nate solve the mystery before Rosamond begins something with international implications?
If you like mysteries, palaces, kings, smorgasbords, trolls, and anything Scandinavian, this super cute book is for you!