My Darling Duke by Stacy Reid

Miss Katherine Danvers has always been a wallflower. But now, with her family on the brink of financial ruin, she finds herself a desperate wallflower. To save her family, she’ll do anything. Luckily, she has the perfect plan…
She’ll impress the ton by simply announcing she is engaged to the reclusive and mysterious Duke of Thornton, Alexander Masters, and secure strong matches for her sisters. No one has heard from the duke in years. Surely he’ll never find out before her sisters’ weddings, and she can go back to her own quiet life.

Soon, though, everything is out of control. At first, it’s just a few new ball gowns on the duke’s accounts. Then, it’s interviews with reporters eager for gossip. Before she knows it, Katherine has transformed herself into Kitty Danvers, charming and clever belle of the ton—with everyone eager to meet her thankfully absent fiancé.
But when the enigmatic Alexander Masters suddenly arrives in the city, dashing and oh so angry, he demands retribution. Except not in the way Katherine expected…