The Spy Who Loved Her by Aydra Richards

Lord Rafe Beaumont has loved Lady Emmeline Prescott for nearly half his life. She is the widow of his former friend, his younger sister’s closest companion…and she doesn’t even know he exists. For years he has contented himself with his position at the very periphery of her life, knowing it is the closest he can ever allow himself to be.
Emma has made a life for herself taking in and raising London’s less fortunate children. They fill the void left by the children she was never able to have—but she is still lonely. Though she has no desire to remarry, she resolves to take a lover to fill her empty nights. Someone she doesn’t know; someone who poses no risk to her fortune or her reputation.