The Duke’s Enforcer by C.H. Admirand

The Duke’s Guard: The men in black who have sworn an oath to protect the Duke of Wyndmere and his family from enemies that would stop at nothing to discredit and destroy him!
Darby Garahan’s latest assignment, to uncover the source behind the threat to kill the duke’s brother-in-law, is complicated when the angel of the streets asks for his help. He must rescue a young woman who has been lured to London with the promise of employment—only to end up at a boarding house rumored to be a front for a brothel.
Aimee Anderson answers an advert for a position at a milliner’s shop in London, taking charge of her life after a lord compromised her on the way to Gretna Green a few years earlier, leaving her stranded at an inn.