Mentor to the Marquess by Melissa Kendall

She taught him about pleasure. He taught her about love.

A survivor of spousal abuse, matchmaker Olivia Heather, the Dowager Countess of Allen, devotes her time to preventing the young ladies of society from following in her footsteps—until a series of newspaper articles accuse her of murdering her late husband.
Her plan to use her feminine wiles to convince the man responsible for the attacks to stop is stymied when the man in question, the infuriatingly handsome Thellusson Vaith, Marquess Lowell, requests that she find a love match for his daughter.
Nonetheless determined to make him pay for his misdeeds, Olivia vows to ignore the attraction simmering between them—until she learns that he hasn’t bedded a woman since his wife’s death nearly twenty years prior. Unable to resist the sensitive, guilt-ridden marquess, she takes on a new role: Mentoring him in the ways of pleasure.