The Art of Victory - Martha Keyes

Diana Donovan was raised aboard a naval ship on a steady diet of war strategy, so when her orphaned friend, Lucy, is denied a perfectly respectable love match by her controlling guardian, Diana sees an opportunity to put her battle knowledge to use in service of a deserving friend.
But Marmaduke Russell is not the middle-aged curmudgeon Diana is expecting. He is a maddeningly cool, unmarried gentleman who seems uniquely able to parry her tactics-and infuse her with even more determination to win. As battle ensues, though, it becomes clear that there may be more hearts in the balance than just Lucy's.