On the Way to the Wedding by Julia Quinn (Bridgertons #8):

A funny thing happened…

Unlike most men of his acquaintance, Gregory Bridgerton believes in true love. And he is convinced that when he finds the woman of his dreams, he will know in an instant that she is the one. And that is exactly what happened. Except…

She wasn’t the one. In fact, the ravishing Miss Hermione Watson is in love with another. But her best friend, the ever-practical Lady Lucinda Abernathy, wants to save Hermione from a disastrous alliance, so she offers to help Gregory win her over. But in the process, Lucy falls in love. With Gregory! Except…

Lucy is engaged. And her uncle is not inclined to let her back out of the betrothal, even once Gregory comes to his senses and realizes that it is Lucy, with her sharp wit and sunny smile, who makes his heart sing. And now, on the way to the wedding, Gregory must risk everything to ensure that when it comes time to kiss the bride, he is the only man standing at the altar…