One Dark Window - Rachel Gillig

For fans of Uprooted and For the Wolf comes a dark, lushly gothic fantasy about a maiden who must unleash the monster within to save her kingdom. But the monster in her head isn't the only threat lurking.
Elspeth Needs a Monster. The Monster Might Be Her.
An ancient, mercurial spirit is trapped inside Elspeth Spindle's head - she calls him the Nightmare. He protects her. He keeps her secrets. But nothing comes for free, especially magic.
When Elspeth meets a mysterious highwayman on the forest road, she is thrust into a world of shadow and deception. Together, they embark on a dangerous quest to cure the town of Blunder from the dark magic infecting it. As the stakes heighten and their undeniable attraction intensifies, Elspeth is forced to face her darkest secret yet: the Nightmare is slowly, darkly, taking over her mind. And she might not be able to stop him.