The Final Enemy by Alanna Knight
A New Case for Inspector Jeremy Faro, Edinburgh's Victorian Detective; Jeremy Faro is hoping for a pleasant break from the demands of his job when he takes a trip across Germany with his writer-companion Imogen Crowe, but it is not to be. The life of the heir apparent of Luxoria is threatened, and Faro has no choice but to become involved. The boy is at boarding school in Scotland and Faro is delegated, by no less than royal command, to restore him to his troubled kingdom. Dangers abound in the hazardous journey back. And to make matters worse, the Inspector knows a secret concerning the boy's birth, a secret relating to the time of the Grand Duchess's brief refuge in Edinburgh, a secret that could destroy them all.