The Tin Princess by Philip Pullman

It is 1882 Sally Goldberg is away to America with her new husband leaving young Jim Taylor, consulting detective, to take the helm in the fourth Sally Lockhart mystery.

In pursuit of a lost girl Jim finds a princess: Adelaide was last seen escaping from the fire that killed Sally's love Frederick.  Now she's engaged to be married to Prince Rudolf, Crown Prince of the tiny kingdom of Razkavia and there are plenty of forces arraigned against her.  Jim finds himself assigned as her protector as they travel with Razkavian exile and interpreter Becky Winter to the country, bordered and threatened by Austria and Germany who both have their eyes on the prize of Razkavia's tin mines.  Murder, skulduggery, high politics and the transformation of a cockney prostitute to a Crown Princess makes for a hell of a ride of a story, entertaining and ultimately heart warming.