A Darker Shade of Magic - (Shades of Magic #1) - V.E. Schwab

Kell serves the Maresh Empire--Red London, as he calls it--as an official ambassador, traveling between the frequent bloody regime changes in White London and the court of George III in the dullest of Londons. After one such visit goes awry, Kell escapes his home for Grey London and runs into Delilah Bard, a cut-purse with lofty aspirations. Now perilous magic is afoot, and treachery lurks at every turn. To save all of the worlds, they'll first need to stay alive.

Book by author V.E. Schwab.