Too Late by Colleen Hoover

Sloan is trapped in a toxic relationship with Asa Jackson, a violent and manipulative drug dealer. She stays with him to protect her mentally challenged brother, who depends on the care Asa provides. Despite the abuse and danger, Sloan feels she has no way out.

Then she meets Carter, a mysterious new man in Asa’s circle—who turns out to be an undercover DEA agent. As Carter falls for Sloan, he risks everything to protect her and bring Asa down. But Asa’s obsession with Sloan turns deadly, and escaping him becomes a life-or-death game.