A plain "Mr.," will do nicely
Just because you are born the daughter of an Earl doesn't mean you must marry a man with a title! Rosabel is the oldest of the Earl of Shelton's daughters. Smart, well spoken, and determined to have as wonderful a marriage as her parents, she has worked hard to be the perfect example for her younger sisters (her next oldest sister, Hilaria, has been heard to say she'll only marry a duke!) Unfortunately, as so often happens in the gossip of the ton, the desire for a duke has been attributed to Rosabel!
James, the Duke of Wexford, works for the Prince, having been given his late father's responsibilities. With a fellow lord once again trying to stir up trouble in the House of Lords, Wexford is following all leads to catch the crooked lord. When James spots the suspect with Rosabel, he begins to suspect that the young lady might somehow be involved with the wretch.
You'll want to stay up late to find out why the bad lord is seeking to cause problems, and if, in the midst of the confusion surrounding the investigation, James and Rosabel can see their way to understand each other's feelings!