I received a review copy from the publishers in exchange for an honest review. It has not affected my opinions.

Genre: Fantasy Age Range: Adult Star Rating: 4 stars Series: yes - first book
Blurb:

Elspeth Spindle needs more than luck to stay safe in the eerie, mist-locked kingdom of Blunder—she needs a monster. She calls him the Nightmare, an ancient, mercurial spirit trapped in her head. He protects her. He keeps her secrets.
But nothing comes for free, especially magic.
When Elspeth meets a mysterious highwayman on the forest road, her life takes a drastic turn. Thrust into a world of shadow and deception, she joins a dangerous quest to cure Blunder from the dark magic infecting it. And the highwayman? He just so happens to be the King’s nephew, Captain of the most dangerous men in Blunder…and guilty of high treason.
Together they must gather twelve Providence Cards—the keys to the cure. But as the stakes heighten and their undeniable attraction intensifies, Elspeth is forced to face her darkest secret yet: the Nightmare is slowly taking over her mind. And she might not be able to stop him.
Blurb taken from Goodreads. Add to your shelves here.
Blurb:
ONE DARK WINDOW is a gothic fantasy about mist-choked kingdoms, magic that comes with a price, and monsters lurking in the back of the mind. Elspeth has accidentally hosted a monster in her mind for eleven years, talking to a creature who can give her powers but clearly has an agenda of its own. Think Venom meet FOR THE WOLF and you’ve got an idea of what this book is like.
The book involves mystery elements about who the monsters in Elspeth’s head and how it was all linked to the cards. There are lots of hints along the way that you could put together, but the focus is on how it builds the sense of unease and atmosphere, rather than the focus being on the mystery.
The magic system of cards was my favourite part of the book. I love the idea of magical cards with varying numbers and powers, all of which come back to bite the user with a heavy cost. They let anyone use them, because of the sacrifice of the mythical Shepherd King who bargained away parts of him to create them, which means that it becomes a race for power to control them. Nobles will pay for them and trade for favours, and the king holds a monopoly to make his elite guards all the more powerful.
Not only do the cards extract a heavy price from the user but, by and large, they are not a nice magic to use on others. Forcing them to tell the truth. Controlling them physically. Invading their minds. This is a magic system with teeth, which really helps to give the world its dark, dangerous feeling.
The book ends with a pay off to the set up, taking the book to the darker ending promised by the set up, which means it feels satisfying to get to that stage. It also sets the stage for an interesting second book.