Thief of Night by Holly Black

Night is made of shadows and shadows have teeth.

Charlie Hall, glue trap for disaster, crooked from the day she was born, who’d never met a bad decision she wasn’t willing to double down on, may have finally met her match.
After defeating Salt and stealing her lover—the powerful shadow, Vince—back from under the noses of the powerful Cabal leaders, she thought she’d won. Agreeing to become the Hierophant and risking her life hunting down dangerous rogue shadows seemed a small price to pay if she got to do it while being tethered to Vince.
But Vince is no longer the man she loved. The tethering process wiped away his memories of the last year, leaving only Red—the ruthless shadow who spent years as a killer for a cruel, mercurial billionaire. Red doesn’t remember Charlie and doesn’t like her.
So when Charlie is ordered to track down the Blight responsible for a massacre at a local church, she’s worse than just alone, heartbroken, and outmatched. She’s vulnerable. In a world where shadows have wills of their own, the question isn’t just how to survive. It’s how to protect yourself from the on thing you can never escape—your own shadow.