The Devil's Only Friend by Dan Wells

John Cleaver, a diagnosed sociopath who channels his darker impulses into catching killers, is pulled into a new, brutal case that blurs human wickedness and supernatural menace. As he hunts a clever and sadistic opponent, he forms uneasy alliances with investigators and other troubled allies while confronting a malevolent force that challenges everything he thought he knew about monsters—and himself. The investigation forces him to face how far he’s willing to go to stop evil without becoming the very thing he fights.

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