King and Joker (Vintage Paperback) by Peter Dickinson

Princess Louise and her father, King Victor II of England, agree that life has become painfully dull. When she’s not in school, Louise spends her days roaming the palace and fulfilling her royal duties while her father fusses over budgets and attempts to keep his family out of the tabloids. So when a prankster begins placing frogs on the breakfast trays, Louise delights in the break from routine—as does King Victor. But this innocent mischief soon escalates into bloodshed when a body is found in the palace.

In an attempt to quell his family’s panic, King Victor resolves to catch the killer. At last he has a purpose—but the palace may be in greater danger than either he or Louise suspects.