My Inconvenient Duke by Loretta Chase

Of all the dukes in all the world, why does it have to be him?

Lady Alice Ancaster needs a husband, and fast, because her reckless brother is going to get himself killed, leaving the dukedom—and her future—in their repellent cousin’s clutches.
The Duke of Blackwood has known Alice since childhood, and they’ve always had a special connection. But years ago he broke it, when he chose a riotous life with his two best friends instead of a reasonable one with her.
The trouble is, the tall, dark, sardonic rogue keeps turning up exactly when needed, and ready—though he sometimes needs a push—to play the hero, if only for as long as it takes.
Being irresistibly drawn to the Wrong Man is not convenient, but when events come to a crisis, Alice has to make a choice. The question is, can she live with it?