Of Fate and Phantoms: The Ministry of Curiosities, book 7

As Charlie and Lincoln’s lives settle into a harmonious pattern, a new threat arises that could have far reaching effects for the royal family and Lincoln’s parents.

When a seer warns the Prince of Wales that his life is in danger, only Charlie and Lincoln take her seriously. After all, what sensible person would believe a gypsy who claims the danger will come in the form of the prince's dead father? 

A meeting at the palace sets them on a path that takes Charlie back to her old haunts in the London slums, on the hunt for a man who can change his appearance. The new threat is their most dangerous yet, but for Lincoln, the biggest concern isn't confronting the shape-changer or meeting his parents for the first time, it's whether Charlie will put his engagement ring on her finger.