A Darker Shade of Magic (Shades of Magic #1)

Prepare to be dazzled by a world of parallel Londons—where magic thrives, starves, or lies forgotten, and where power can destroy just as quickly as it can create.

Kell is one of the last Travelers-magicians with a rare, coveted ability to travel between parallel universes-as such, he can choose where he lands.

There's Grey London, dirty and boring, without any magic, with one mad king-George III. Red London, where life and magic are revered-and where Kell was raised alongside Rhys Maresh, the rougish heir to the throne. 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, amdassador of the Maresh empire, carrying the monthly correspondences between the royals of each London. Unofficially, he's a smuggler, a dangerous, defiant hobby to have-as proven when Kell stumbles into a setup with a forbidden token from Black London.
Fleeing into Grey London, Kell runs afoul of Delilah Bard, a cut-purse with lofty aspirations, who first robs him, then saves him from a dangerous enemy, and then forces Kell to spirit her to another world for a proper adventure.

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