A Darker Shade of Magic - V. E. Schwab

Most people only know one London; but what if there were several? Kell is one of the last Travelers - magicians with a rare ability to travel between parallel Londons. There's Grey London, dirty and crowded and without magic, home to the mad king George III. There's Red London, where life and magic are revered. Then, White London, ruled by whoever has murdered their way to the throne. But once upon a time, there was Black London...