Writer's Muse

On his father's death, Maddie found the typewriter and letters of his father to someone starting from 50 years ago. His father left it to him and asked her to read the letters everyday. As she read those letters, Maddie found that the receiver of the letter was the same with the female characters of his father on all his novels. Noticing these, Maddie found herself stuck with so many questions. Will the letters unfold who was the real muse of his father on all his novels?