An Offer From a Gentleman by Julia Quinn
Sophie Beckett, an illegitimate daughter of an earl, plays servant to her wicked stepmother and stepsisters. One night, Sophie sneaks out to attend a masquerade ball at the house of the Bridgertons, a wealthy, popular family within London society. When she meets Benedict Bridgerton, they quickly become enchanted with one another during the magical evening. But when the clock strikes midnight, Sophie disappears, leaving Benedict with no clue what she looks like or who she is.