Shades of magic

Kell is one of the last Antari, magicians with the rare ability to travel between parallel universes, and his duty is to serve as an ambassador between them. There is Grey London, dirty and boring and without magic; Red London, where life and magic are revered; White London, a starving world ruled by whoever has murdered their way to the throne; and once, there was Black London. Officially, Kell is the personal messenger of the royal family of Red London, but unofficially, he is a smuggler, servicing people willing to pay for even the smallest glimpses of a world they will never see. It is a defiant hobby with dangerous consequences.

After a smuggling deal goes awry, Kell escapes to Grey London where he runs into Delilah Bard, a cut-purse with lofty aspirations. She first robs him, then saves him from a deadly enemy, and then forces him to take her with him for a proper adventure. Now, a dangerous magic is afoot, and treason lurks at every turn. To save all of the worlds, Kell and Lila will first have to stay alive. This journey will challenge their limits and force them to confront the darkness in their pasts.