Unseen. Shaye Archer Series, #5

Madison Avery looks down from her penthouse as a light flicks on in the building across the street, and a man and woman enter the vacant apartment. As the woman steps onto a large blue rug, the man raises his hand to the back of her neck. The woman collapses, and Madison sucks in a breath. The man checks for a pulse then reaches into his pocket. He must be calling for help. But then Madison sees the glint of the knife. She grabs her cell phone and dials 911.
If the police don't catch him now, she can't help them later. Her prosopagnosia-facial blindness-won't allow her to identify the man. Or his victim. But the killer doesn't have the same limitations. He looks up and stares directly at her.