I heard about this series from the Fated Mates podcast because Sarah MacLean described the plot of the second book as a brother’s best friend who comes to the defense of a ruined lady only to discover that the rumors of her lost virtue are actually true! 
This first book concerns that brother himself, who is on a mission to find his father’s former mistress in order to save his sister’s dowry. Turns out, the best person to help with the job is the mistress’s niece, who just so happens to be the woman John had not stopped thinking about since their rendezvous in a garden under secret identities. The two rivals—divided by history of family scandal—travel through England to find the mistress and fall deeply in love along the way. 

🌶️🌶️🌶️🌶️