Duke of Midnight (Maiden Lane #6) by Elizabeth Hoyt

Twenty years ago Maximus Batten witnessed the brutal murders of his parents. Now the autocratic Duke of Wakefield, he spends his days ruling Parliament. But by night, disguised as the Ghost of St. Giles, he prowls the grim alleys of St. Giles, ever on the hunt for the murderer. One night he finds a fiery woman who meets him toe-to-toe—and won't back down . . .