M is for marquess / Grace Callaway.

Enhanced descriptions from Syndetics:

A Prisoner of His Past

Gabriel Ridgley, the Marquess of Tremont, is dubbed The Angel for his pristine reputation. Yet he harbors a dark secret: a spy during the Napoleonic Wars, he bears deep and dangerous scars. When he meets Miss Dorothea Kent, a delicate virgin, he is torn between carnal desire and the need to protect her from his inner demons.

A Captive of Her Passions

Despite a frail constitution, Thea Kent has strength of heart and spirit. A wild attraction blooms between her and Gabriel, a widowed aristocrat with tortured eyes, but he resists the pull between them. When fate places his disabled son in her care, she's determined to safeguard the boy and unravel the mystery of Gabriel's past.

Freed by Love and Passion

Someone is killing Gabriel's former colleagues, and he may be next on the list. As the desperate hunt for the villain begins, he and Thea get entangled in intrigue, betrayal, and desire too powerful to resist. Will trust help them to survive ... and true love set them free?

Books by Grace Callaway:

The Heart of Enquiry series (hot historical romance series starring the Kent family):
Prequel Novella: The Widow Vanishes
Book 1: The Duke Who Knew Too Much
Book 2: M is for Marquess
Book 3: The Lady Who Came in from the Cold
Book 4: The Viscount Always Knocks Twice
Book 5: Never Say Never to an Earl
Book 6: The Gentleman Who Loved Me (Summer 2017)

The Mayhem in Mayfair series (hot historical romance series):
Book 1: Her Husband's Harlot
Book 2: Her Wanton Wager
Book 3: Her Protector's Pleasure
Book 4: Her Prodigal Passion

Chronicles of Abigail Jones (hot historical paranormal romance series):
Book 1: Abigail Jones
Book 2: Abigail Jones and the Asylum of Secrets (2017)