The Duke Always Wins by Jessie Clever

Lady Anna Elmont, the dowager Countess of Wexford, has only one wish: to remain safely a widow. Having spent two years under the control of her manipulative husband, Annie plans to enjoy widowhood and discover who she might have been had she not wed.

If the Duke of Grimsby will let her.
Despite their difference in age or perhaps because of it, Gabriel Phelps, the Duke of Grimsby is compelled to protect his best friend’s widow. After witnessing his mother fall victim to a man with dishonorable intentions after she was widowed, Gabriel will not allow the same thing to happen to Annie and so he proposes only to be flatly refused.
But when a compromising situation forces Gabriel to act, Annie must submit if only to save her family’s reputation and her sister’s hope for a future.
Will she be trapped in another cold and controlling marriage? Or will the unexpected spark of attraction between them lead to something of which she’d never dared dream?