The Dream of a Duchess. The Widowers of the Aristocracy, #1

When Lady Isabella thinks she has witnessed the murder of her mother-by her father, the Earl of Craythorne-she rides off to London in search of the man her mother, Arabella, instructed she contact lest anything befall her. Directions send Isabella to the doors of The Elegant Courtesan, a high-end brothel owned by David Fitzwilliam, Earl of Norwick, to whom she relates her gruesome tale. Stunned and heartbroken at hearing the fate of Arabella, David vows to ensure Isabella's safety from Craythorne, and he enlists the aid of Octavius, Duke of Huntington, to provide protection and a home in the country, where she won't be found.
He would do it himself, but he's due to marry the girl's aunt, Lady Clarinda, and has some courting to do- his twin brother seems determined to make her his own wife. Still suffering from the loss of his wife in childbirth, Octavius has long since given up the joy he took in his race horses-choosing to ignore his stables and country estate and live in London, immersed in reading and attending sessions of Parliament.
Not particularly pleased with having a ward, Octavius soon discovers Isabella's passion for horses goes far beyond just riding them. With a new generation of racehorses in his stables, and his country estate suddenly in pristine condition, he may just experience some horse sense himself in "The Dream of a Duchess".