A Darker Shade of Magic by V. E. Schwab — book slip

Kell is one of the last Antari—magicians with a rare, coveted ability to travel between parallel Londons: Red, Grey, White, and, once upon a time, Black.

Raised in Red London, Kell serves the Maresh Empire as an ambassador, but he also engages in smuggling magical artifacts between worlds.

When a deal goes wrong, Kell finds himself in Grey London, where he meets Delilah Bard, a daring thief with dreams of adventure. Together, they must navigate treacherous magic and political intrigue to save the worlds from a looming threat.

First in the Shades of Magic series by V. E. Schwab.