Oblivion's Hymn (Divine Songs #1) by AJ Peterson:
 
In a world where magic is ruled by bargains, how much would you pay for the thing you want most?

Echo’s shapeshifting has been a curse all her life. The secret police hunt her for her magical blood, and the rest of the world believes she’s a monster. To prove them wrong and escape capture, Echo bargained with a god to become a famous bard. But when she didn’t uphold her end of the deal, he took her singing voice away. Now the police are closing in, and her childhood friend, Rhienne, is the only one loyal and reckless enough to help get her voice back.

But Rhienne’s lust for power has her in trouble of her own. Her mother is dead, her scheme to steal magic from the head of the secret police failed, and time is running out before the gods come to collect her soul. If she can’t resurrect her mother—who has a history of finding contract loopholes—Rhienne belongs to the God of the Void forever. Using Echo’s innate talents might be Rhienne’s last chance at freedom.

Echo and Rhienne will search continents and dimensions for power strong enough to thwart their enemies and undo their mistakes. But the gods have their own plans to maintain balance. One way or another, they always get what they’re owed.