A previously unknown collection of wartime letters from Gen. Dwight D. Eisenhower to his driver, Capt. Kay Summersby, appears likely to stir renewed debate over whether the two were lovers during the last year of World War II. Both General Eisenhower, who was married, and Captain Summersby initially denied the long-rumored romance. But in 1976, as she was dying of cancer, Miss Summersby published a second book of memoirs of the war years, "Past Forgetting: My Love Affair With Dwight D. Eisenhower," in which she described a passionate but frustrating affair with the Supreme Allied Commander.