Wayfarer by Alexandra Bracken

All Etta Spencer wanted was to make her violin debut when she was thrust into a treacherous world where the struggle for power could alter history. After losing the one thing that would have allowed her to protect the Timeline, and the one person worth fighting for, Etta awakens alone in an unknown place and time, exposed to the threat of the two groups who would rather see her dead than succeed. When help arrives, it comes from the last person Etta ever expected—Julian Ironwood, the Grand Master’s heir who has long been presumed dead, and whose dangerous alliance with a man from Etta’s past could put them both at risk.

Meanwhile, Nicholas and Sophia are racing through time in order to locate Etta and the missing astrolabe with Ironwood travelers hot on their trail. They cross paths with a mercenary-for-hire, a cheeky girl named Li Min who quickly develops a flirtation with Sophia. But as the three of them attempt to evade their pursuers, Nicholas soon realizes that one of his companions may have ulterior motives.

As Etta and Nicholas fight to make their way back to one another, from Imperial Russia to the Vatican catacombs, time is rapidly shifting and changing into something unrecognizable… and might just run out on both of them.