The Reluctant Widow — Georgette Heyer

A fateful mistake... Stepping into the wrong carriage at a Sussex village, Elinor Rochdale is swept up in a thrilling and dangerous adventure. Elinor is rather surprised that her prospective employer is quite rich, and more so at tile decayed grandeur of the house to which she is transported. Elinor thought she was entering Highnoons as a governess, but Eduard Carlyon, the handsomely fashionable gentleman who carefully interviewed her obviously needed no governess. Carlyon was seeking a wife-not for himself, but for his young cousin, Eustace Cheviot, the dissipated and profligate owner of the ruined estate, who now lay on his deathbed. Surprised and repulsed as she was by this strange proposal, Elinor was nevertheless unable to resist Carlyon and his mystifying plans. Ned persuades Elinor to marry his cousin as a simple business arrangement.

A momentous decision... And so in the short span of a few hours would-be governess became a bride - and a widow almost as soon as the ring is on her finger. Even as she assumed the duties of mistress of Highnoons, even as she tried to solve the puzzle that placed her in this unusual situation, Elinor found herself inextricably- drawn to Ned Carlyon, the aloof, caustic man who treated her with nothing more than cousinly respect - the last thing that spirited Elinor wanted.... As partner in a secret conspiracy to save a family's name she finds herself embroiled with uninvited guests, housebreakers, missing government papers, an Napoleonic spy ring, and a shocking murder. And mysterious conspirator Lord Carlyon won't let her leave....