A Lord Undone by Rose Pearson

Lady Beatrice Thornton is surrounded by gossip.

Her drunken father has made it known that there is some question over the legitimacy of her birth and now she can barely lift her head in society. Desperate for aid, she is more than grateful for the assistance of Lady Smithton, who promises that she will help her find a husband.
Whilst seeking solace in a bookshop one afternoon, Beatrice is astonished to discover a mysterious note slipped in between the pages of a book. Knowing she should not keep it, she cannot help but take it home, hoping that it will take her mind from her difficult circumstances.
Frederick, Earl of Greaves, is a stickler for propriety. It is all the more irritating to him that he has had to step into an unscrupulous situation in order to rescue his brother. What makes matters worse is that the last note has somehow gone missing and that, somehow, a young lady has become entwined in it too!
Determined he will have to remove her from this situation at once, Fredrick goes in search of her and discovers that he may have to involve himself with her more fully than he ever intended.