The Timekeepers' War by S.C. Jensen

Ghost.
She can find anyone, for the right price--anyone, except her sister.

Lyca.
Ghost searches for her in every dark and desperate corner of the City. But her so-called life changes when she returns home to find a mysterious stranger with an offer to make history...and to find her sister.

Lynch.
A terrorist. One of the most feared figures in the City--and he needs Ghost's help. Lynch is out to infiltrate, and destroy, the oppressive Ursaar Empire, a scaffolding of untouchable, glittering bridges and buildings above the City's surface. He offers Ghost a chance to join his cause, to be a part of history in the making, and--more importantly to Ghost--his aid in finding Lyca, ten years gone. Between Lynch, Ghost, and an enigmatic group of scholars called The Timekeepers, the City will soon be reborn.

But Lynch's plan means war. If Ghost agrees, she must delve deep into the heart, and darkness, of the City and its truths--and her own. Lynch will change everything she thought she knew about their world. How far will her search for Lyca take her? And can Ghost live with who she has become once she finds her?