Mine Till Midnight (The Hathaways, Book 1) / Lisa Kleypas,

When an unexpected inheritance elevates her family to the ranks of the aristocracy, Amelia Hathaway discovers that tending to her younger sisters and wayward brother was easy compared to navigating the intricacies of the ton. Even more challenging: the attraction she feels for the tall, dark, and dangerously handsome Cam Rohan.

Wealthy beyond most men's dreams, Cam has tired of society's petty restrictions and longs to return to his "uncivilized" Gypsy roots. When the delectable Amelia appeals to him for help, he intends to offer only friendship―but intentions are no match for the desire that blindsides them both. But can a man who spurns tradition be tempted into that most time-honored arrangement: marriage? Life in London society is about to get a whole lot hotter….in Lisa Kleypas's Mine Till Midnight.