Deeplight by Frances Hardinge
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The gods are dead. Decades ago, they turned on one another and tore each other apart. Nobody knows why. But are they really gone forever? When 15-year-old Hark finds the still-beating heart of a terrifying deity, he risks everything to keep it out of the hands of smugglers, military scientists, and a secret fanatical cult so that he can use it to save the life of his best friend, Jelt. But with the heart, Jelt gradually and eerily transforms. How long should Hark stay loyal to his friend when he’s becoming a monster—and what is Hark willing to sacrifice to save him?