Love Letters from a Duke by Elizabeth Boyle

Mistaken identity to the extreme. Felicity Langley thinks the handsome man on her front porch is the new footman, not her presumed fiance. While she falls more and more in love with him, the Duke of Hollindrake is trying to figure out if she's really the woman she seems to be, not the schemer he had first believed (his grandfather had corresponded with Felicity making her believe that she was engaged to the current duke which he doesn't know about.