The Duke's Unexpected Bride — Rachel McNeely

This book was previously published as: The Duke's Dilemma.
After losing her fiancé in a war, Helena Steeples vowed never to wed. But when her twin, Mary Ann, refuses the Duke of Monteroy, Nicholas Selwyn's proposal, Helena finds herself attracted to him.
Nicholas knows Helena, the shy twin, does not meet his initial criteria for a wife, but she fascinates him. With both families' support, the duke pursues Helena and she signs a contractual agreement to marry him.
Shortly before their wedding, her former fiancé, believed dead, returns, postponing the nuptials. Helena notices changes in her former beau, while the duke suspects he's the traitor they've long hunted.
Helena finds herself in the middle of an attempted murder, treason, and a former fiancé who wants to hold her to their earlier obligation. He knows Helena's secret and can destroy her reputation and ruin her wedding plans to Nicholas.
Once the secrets are revealed, will Helena marry the duke or her former fiancé?