How does their relationship develop during the course of the novel?

I assume you are referring to the relationship between Maxim and the narrator. Their marriage is based upon love and mutual respect. Over the course of the novel, their love is challenged by Maxim's past. The narrator feels threatened by Maxim's late wife and believes that he still loves her. She is also surrounded by Rebecca's loyal servants. As the mystery about Rebecca's death is revealed, Maxim makes a startling confession, but rather than pull away, the narrator is freed from all of her fears. Their marriage is a happy one, though it takes traversing through the past to find a happy present.