For the Sake of a Scottish Rake by Anna Bradley

After a sheltered upbringing, Lady Lucinda Sutcliffe is finally embarking on her first season, eager to experience everything she’s missed. When Lucy realizes that her uncle plans to quickly marry her off in exchange for a slice of her fortune, she begs a favor of a new acquaintance, Ciaran Ramsey. If Lucy remains single until she turns twenty-one, she–and her money–will be out of her uncle’s power. All the charming Scot needs to do is woo her for six weeks, and then jilt her.
The Ramseys don’t need the scandal of a false engagement attached to their name. But Lucy’s older suitor is both distasteful and dangerous, and Ciaran can’t allow his lovely friend to be forced to marry such a man. And besides, the more time Ciaran spends with his new “betrothed,” the more their ruse begins to feel very much like the real thing. Passion like this is impossible to feign, but how much is a rogue willing to risk for love?