"A Marquess, a Miss and a Mystery" by Annie Burrows is a Regency Romantic thriller set in about 1814, during the period after Napoleon's initial defeat, when he was exiled to the island of Elba but before the "hundred days" when he attempted a comeback, culminating in the battle of Waterloo.
At the start of the novel the heroine, Horatia Carmichael, is trying to make contact with dashing rake Lord Devises, who had been the best friend of her murdered brother. She knows the Marquess and her brother had been working for the government to track down a suspected spy ring, who had killed her brother when he got too close to them, and believes she has information which may make it possible to catch the culprits.
What follows is pretty much standard for the sub-genre of regency romantic detective or spy novels. The characters are reasonably well crafted enough to be interesting and there are a few moments of very amusing British comedy.
The espionage sub-plot is not as well crafted as the romance and the hero's title is not well explained.
As the younger son of a duke and half-brother of the current holder of the dukedom, the hero of this story would normally be addressed by the courtesy title of "lord" followed by his Christian name and family name: but would not usually be a Marquess unless he had inherited that title through his mother, which would be very rare.
There are some UK peerages which can descend through the female line, although when I looked them up on the Debrett's website none were Marquessates, though one of the subsidary titles (in the rank of baron) currently held by the 8th Marquess of Anglesea apparently could be so inherited.
If you like the regency romance/detective sub genre you will probably find reading this tale a reasonably amusing way to spend an hour or so.