House of Earth and Blood by Sarah J. Maas

Bryce Quinlan had the perfect life-working hard all day and partying all night-until a demon murdered her closest friends, leaving her bereft, wounded, and alone. When the accused is behind bars but the crimes start up again, Bryce finds herself at the heart of the investigation. She'll do whatever it takes to avenge their deaths.

Hunt Athalar is a notorious Fallen angel, now enslaved to the Archangels he once attempted to overthrow. His brutal skills and incredible strength have been set to one purpose-to assassinate his boss's enemies, no questions asked. But with a demon wreaking havoc in the city, he's offered an irresistible deal: help Bryce find the murderer, and his freedom will be within reach.