Conjuring Of Light - V.E. Schwab

The third book in the series set in a world where magicians travel between parallel London.

The precarious equilibrium among the four Londons has reached its breaking point. Once brimming with the red vivacity of magic, darkness casts a shadow over the Maresh Empire, leaving a space for another London to rise. Kell-once assumed to be the last surviving Antari-begins to waver under the pressure of competing loyalties.

Lila Bard, once a commonplace-but never common-thief, has survived and flourished through a series of magical trials. But now she must learn to control the magic, before it bleeds her dry. Meanwhile, the disgraced Captain Alucard Emery and the Night Spire crew are attempting a race against time to acquire the impossible, as an ancient enemy returns to claim a crown and a fallen hero is desperate to save a decaying world...