Dukes do it better
(Book)

Series numeration from goodreads.com.

After a debut season plagued by scandal, Lady Emma Hardwick is ready to return to London, now with her young son in tow, and make a match. She's looking for someone respectable. Someone wholly unlike Malachi Harlow, the new Duke of Trenton and former ship captain, whose long hair and tattoos make him decidedly dangerous to her peace of mind. Malachi would rather be at sea than in a London ballroom. But until he can sort out why the admiralty brought him home, he has to stay landbound. That becomes less of a hardship when he meets the beguiling Lady Emma, whose dimples and easy laughter capture his imagination. When they start receiving threatening notes, they realize that there's more to their connection than chemistry, and they'll have to work together to figure out why someone wants to ruin their lives. -goodreads.com.