A Darker Shade of Magic by Victoria Schwab

Kell is one of the last travelers–magicians with a rare, coveted ability to travel between parallel universes connected by one magical city.

There’s Grey London, dirty and boring, without any magic, and with one mad King–George III. Red London, where life and magic are revered–and where Kell was raised alongside Rhy Maresh, the roguish heir to a flourishing empire. White London–a place where people fight to control magic and the magic fights back, draining the city to its very bones. And once upon a time, there was Black London. But no one speaks of that now.

Officially, Kell is the Red traveler, ambassador of the Maresh empire, carrying the monthly correspondences between the royals of each London. Unofficially, Kell is a smuggler, servicing people willing to pay for even the smallest glimpses of a world they’ll never see. It’s a defiant hobby with dangerous consequences, which Kell is now seeing firsthand.

Fleeing into Grey London, Kell runs into Delilah Bard, a cut-purse with lofty aspirations. She robs him, saves him from a deadly enemy, and finally forces Kell to spirit her to another world for a proper adventure.

Now perilous magic is afoot, and treachery lurks at every turn. To save all of the worlds, they’ll first need to stay alive.