The Dark Mirror by Samantha Shannon

Everything is about to change.

Paige Mahoney is outside the Republic of Scion for the first time in more than a decade-but she has no idea how she got to the free world. Half a year has been wiped from her memory.
Her journey back to the revolution soon takes her to Venice, where the Domino Programme has uncovered evidence of a secret Scion plan. Before Paige can return to London, she must help the network unravel the sinister Operation Ventriloquist, which threatens to bring Europe to its knees in weeks.
And it soon becomes clear that the one person who could recover her memories-Arcturus Mesarthim-might also hold the key to thwarting Scion, allowing the revolution to strike an unprecedented blow . . .
With its intricate worldbuilding, slow burn romance, and “complex, ever evolving, scrappy yet touching” (NPR) heroine, the Bone Season series shows Samantha Shannon at the height of her considerable powers.