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

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.  Kell was raised in Arnes—Red London—and officially serves the Maresh Empire as an ambassador, traveling between the frequent bloody regime changes in White London and the court of George…