Olivia and the Masked Duke by Grace Callaway 

Spirited debutante Lady Olivia MacLeod, cherished daughter of the Duke and Duchess of Strathaven, has two interests: solving mysteries and winning the love of her secret girlhood crush, the infamous Duke of Hadleigh. When she joins Lady Charlotte Fayne’s charity for genteel young ladies and discovers that it is a front for an investigative agency, she is thrilled and dedicates herself to solving a deadly case. Little does she realize that the path to danger will also lead to her heart’s deepest desire… Read more.