It's a supernatural buddy cop book about a semi-suicidal human bounty hunter (think a female Riggs from Lethal Weapon) and an American-raised Japanese demon called an Oni who looks like a man.
What starts as two separate missing women cases quickly proves they are linked and tie into what they, later find out is a necromantic attack on the United states using a combination of new magi-technology and arcane drug to unleash a curse that will turn a sixth of the population into zombie slaves.