THE LETTERS OF NOEL COWARD, edited with commentary by Barry Day, Knopf, 783 pages, $37.50.

If you're old enough to remember Noel Coward, you know he was an unusually successful British playwright, novelist, producer, director, actor and songwriter. But not everyone who writes well professionally can do it on an everyday basis.

When Coward wrote a letter (almost a lost art today), he was alternately charming, whimsical, witty and entertaining. That is proved by this evocative collection by Barry Day, who has written seven previous books on Noel Coward.

Among the numerous people with whom he corresponded included Gertrude Lawrence, Marlene Dietrich, John Gielgud, Vivien Leigh, Alec Guinness, Virginia Woolf, Winston Churchill, Greta Garbo — and his mother, Violet Coward.

According to Barry Day, these letters are especially precious items because they say as much about the writer of the letters as they do about the relationships. The letters, Day says, "complete the portrait of what it was like to be Noel Coward," including disappointments, youthful hopes, exhilaration with success and the feeling of "being banished to the wings."

To Virginia Woolf, Coward wrote a fan letter in 1928 to praise her new book, "Orlando," in which he said, "If ever I could write one page to equal in beauty your 'Frozen Thames' description ... I should feel that I really was a writer."

She wrote back, saying she had to get someone else to "decipher the signature. When sure that it was yours, my heart leapt that you should have liked that innocent story and I feel — not like a dog — but like a cat that is purring all over with pleasure at your praise and generosity."

Gertrude Lawrence, the actress who co-starred with Coward in his witty play, "Private Lives," wrote him after he opened in

"Cavalcade" in 1931, expressing "humble admiration and complete adoration." She quoted him in saying, "'It's pretty exciting to be English."

Coward replied to her "that you are a chameleon — an elegant one but still a chameleon. You never play a part the same way two nights running but it certainly keeps whoever you're playing with — in this case your author — on his tippy tip toes. And I must also admit — it's pretty exciting to be playing opposite Miss Gertrude Dagmar Lawrence-Klasen and I can't wait until the next time. And I can just hear you saying — 'Well, darling, that's up to you.'"

When, during World War II, Coward wrote a satirical song called "Don't Let's Be Beastly to the Germans," he was criticized in many quarters. When Winston Churchill and Franklin Roosevelt had a summit meeting to form the Atlantic Charter, they argued for some time about which verse preceded another verse in Coward's "Mad Dogs and Englishmen."

When Coward was asked to adjudicate, he said he hated to admit it but "the President was right," to which Churchill replied, "Britain can take it." But Coward evidently lost his opportunity for knighthood when he wrote and produced a film about the war, "In Which We Serve" that was well-received by both British and Americans.

Churchill, however, was not sure about it. Even though the king liked the film, Churchhill wrote him in 1942 recommending against knighthood. But accolades poured in anyway. This book provides a delightful glimpse into his brilliant mind.