Gideon by Ashe Barker

Order of the Black Knights

Gideon Maybury enjoys a life of wealth and privilege, not to mention the advantages his position offers him in his career as a merchant banker and his less public life as a high-class, skilled, and very well-paid assassin for Her Majesty’s government.
When his brother dies unexpectedly, he becomes the Duke of Westmoreland.

Michael Mathison has hated Gideon since they were at university together. He’s convinced Gideon had a hand in the death of Michael’s college lover, Christopher, and that he had something to do with the death of his own brother.
So he gets a job as Gideon’s driver, enabling him to investigate the circumstances surrounding the death of the elder Maybury sibling. At first his suspicions seem to be confirmed, but clues emerge that suggest all is not as it appears at Maybury Hall.

As the mystery deepens, so does the attraction between the two implacable enemies, as does the feeling that they have met before—under dark and terrible circumstances. Each has reasons not to trust the other, but neither is averse to a bit of kinky play.
Gideon and Michael end up owing each other their lives, and it results in consequences neither could have imagined.