The Invocations by Krystal Sutherland

Zara Jones believes in magic because the alternative is too painful to consider-that her murdered sister is gone forever and there is nothing she can do about it. Rather than grieving and moving on, Zara decides she will do whatever it takes to claw her sister back from the grave-even trading in the occult.

Jude Wolf may be the daughter of a billionaire, but she is also undeniably cursed. After a deal with a demon went horribly wrong, her soul has been slowly turning necrotic. It's a miserable existence marred by pain, sickness, and monstrous things that taunt her in the night. Now that she's glimpsed what's beyond the veil, Jude's desperate to find someone to undo the damage she's done to herself.

Enter Emer Byrne, an orphaned witch with a dark past and a deadly power, a.k.a. the solution to both Zara's and Jude's problems. Though Emer lives a hardscrabble life, she gives away her most valuable asset-her invocations-to women in desperate situations who are willing to sacrifice a piece of their soul in exchange for a scrap of power. Zara and Jude are willing, but they first have to find Emer.

When Emer's clients start turning up dead all over London, a vital clue leads Zara and Jude right to her. If a serial killer is targeting her clients, Emer wants to know why-and to stop them. She strikes a tenuous alliance with Zara and Jude to hunt a killer before they are next on his list, even if she can't give them in return what Zara and Jude want most- a sister and a soul.