A Tangle of Gold (the Colors of Madeleine, Book 3)

In Cambridge, England, Madeleine Tully and Elliot Baranski are still working to gather the Royal Family, and get them back to their parallel world--but in Cello, the Hostiles are working against the Royal Family, and the World Severance Unit is trying to stop all contact between the worlds, and it is hard to know who to trust when your only contact is letters slipped through a crack between worlds.