The other Lady Diana's charming love letters; A Durable Fire: The Letters of Duff and Diana Cooper 1913-1950, edited by Artemis Cooper. New York: Franklin Watts. 332 pp. $17.95.

There was a time when the words ''Lady Diana'' signified, not the current Princess of Wales, but Lady Diana Cooper, nee Manners, the glamorous daughter of the Duke and Duchess of Rutland and wife of the brilliant diplomat and writer Duff Cooper.

Unlike the more recent Diana, whose fascination for the public seems based upon her lack of distinguishing characteristics, the first Diana impressed all who knew her as a unique individual: charming, witty, and the most beautiful woman of her generation. While the second Diana seems to serve as a sort of Everywoman, a symbol of the myth that any woman can someday capture Prince Charles, if not Prince Charming, the first Diana led a life that was anything but a textbook role model (as documented in Philip Ziegler's biography ''Diana Cooper'').

These letters, edited and introduced by the Coopers' granddaughter, are essentially love letters, written during the Coopers' courtship and marriage. When Duff was sent to the Western Front in 1918, they wrote each other every day. Duff's reaction to the war differed sharply from that of war poets like Wilfred Owen and Siegfried Sassoon, who were totally repulsed. The war, he felt, was wasteful, loathsome, yet still somehow ''romantic.''

Diana, who worked hard as a wartime nurse in London, was able to spend at least some evenings delighting in her own allure: ''My limbs,'' she wrote Duff, ''in this demi-light seemed to flow with peculiar grace, and my green film cloak to foam and curl around them with Aphrodite's birth-waves. . . . I wanted you to see me, almost as greatly as my eyes wanted you.''

The Coopers's gift for self-dramatization undercut by irony is beautifully epitomized in this passage of Duff's, also written from the Western Front: ''I love you more and more . . . which I could have sworn was impossible - and you must love me more . . . and tell me so continually - and exaggerate a little when you tell me.''

Duff and Diana's determination not to be boring is reflected in their letters. Their great sense of fun has clearly been transmitted to their granddaughter, whose informative notes and commentary are quite as charming as the letters. Of Diana's execrable orthography, she notes, ''I have . . . corrected the spelling - except where it seemed too breathtaking or inspired to change.''