The Trouble with Being a Duke by Sophie Barnes

Sometimes happily ever after…
Anthony Hurst, Duke of Kingsborough knows the time has come for him to produce an heir. But first he must find a bride. When he meets the most exquisite woman at his masquerade ball, he thinks his search is over…until the breathtaking beauty runs off. With few clues other than her figure, her scent, and the memory of her kiss, Anthony must find his mystery lady.

…needs a little bit of help.
Isabella Chilcott can scarcely believe it: she is finally at the Kingsborough Ball. As a child, she dreamed of dancing a waltz here and now, thanks to a gorgeous gown she’s found in the attic, Isabella is living her fairytale fantasy. And she’s waltzing with the Duke of Kingsborough himself! But she must escape before he discovers her secrets…for she is not who she pretends to be, and falling in love with Prince Charming is the last thing she can allow herself to do…