Knight is the fourth book in the Reed Security Series and it is recommended they are read in order.

Knight
I watch her in the darkness. I follow her to work just to make sure she's okay. She saved my life, and ever since then, I haven't been able to get her out of my mind. I would do anything for her, including disposing of anyone that comes near her. I know I'm no good for her. An assassin doesn't have a high life expectancy, nor does the woman he sleeps with. If I stick around, she'll only end up dead. But the longer I'm around her, the more I crave her. And now someone knows about her. I should let her go. I should walk away. But I need her too much. And soon, she'll need me too.

Kate
Had I known that I was taking care of an assassin, I never would have stayed. I never would have wanted to know his secrets. I can feel him everywhere now. He's following me, watching me. I can't help but want to see him again, even though I know he's no good for me. I'll end up dead if I hang around him, but the way my body tingles when he's around tells me that I'm meant to be with him. But how can a doctor ever live in the same world as an assassin?