Another idea - 

(supernatural/fantasy). An ancient book with a demon locked inside. A demon who wants out. It rests in a library of an old estate. Newly bought, staff arrive to begin cleaning for the new owners. He is a shy man. She is a sweet woman. He is there to take care of the library. She is there to be part of the household staff, cleaning and such things.  He is smitten. She is smitten. The book gets found by him and he unknowingly let's the demon find a home in him. The catch, the demon can only come out at night. During the day he is the sweet man she is growing to care for. At night, he becomes someone else.