Half a Dark Heart by H.F. Askwith

The Horrors gain strength when things in our world are broken.

A bone. A promise. A heart.

Ever since she first encountered 'The Horrors', Alice has hidden herself safely behind the walls of her family's Whitby guest house. Only her beloved sister, Lucille, knows why: that Alice can see things - gruesome, terrible things - in the darkest corners of our world. Shadow-monsters which will do anything to satisfy their violent cravings - and seemingly nothing can stop them. Of course, no one would believe them if they told them - and so the girls stay silent.

That is, until Lucille goes missing, and the boy she was in love with turns up dead. All of which coincides with the arrival of Austin - a dashing young actor from London, who claims a demon has stolen his voice, and only Alice can help him.

Faced with a world and powers she never even knew existed, Alice must race against time to save her sister, herself, and perhaps the only other person who's ever cared about her. As well as send a demon or two back to where they came from . . .