Princess Margaret's 1978 divorce from the earl of Snowden was the first in the immediate royal family in four centuries, and her subsequent romance with sometime gardener Roddy Llewellyn angered her sister, Queen Elizabeth, and titillated the readers of Britain's scandal columns.

To put it bluntly, Princess Margaret has been unlucky in love. And now, thanks to a book by London's Daily Mail gossip columnist, Nigel Dempster, the whole world will soon know just how unlucky she's been.

"The marriage that supposedly lasted 18 years actually lasted only five, and she ran off with her daughter's godfather," says Dempster, whose unauthorized biography filled with details of failed romances, H.R.H. The Princess Margaret: A Life Unfulfilled , has been a bestseller in England since last fall. In April, Macmillan publishes the book here; both publisher and author hope Americans enchanted with British royalty after last summer's wedding of Prince Charles and Lady Diane will turn to Dempster's book for their royal fix this season.

Dempster, 40, is London's bad-boy columnist, a bon vivant who relishes dishing the dirt about the private lives of public figures, some of whom cooperate with him. For example, Dempster says her royal highness confirmed her relationship with her daugther's godfather, wine negociant Anthony Barton.

"It's all very French," says Dempster, who notes Barton and his wife are still friends with Princess Margaret. For Dempster, running photos of such fun couples is routine fare.

"My idea is that when you have read one of my items you know who the people are, what their pedigree is, their financial standing, who they have been sleeping with, and who they are currently going to bed with," says Dempster, who began keeping notes of conversations with Princess Margaret after their first meeting at a party in 1970. It was also his good fortune to know Llewellyn--Dempster says he hired him to garden for him several years ago. In 1979, with the idea of selling a series of articles about the strange courtship of Princess Margaret and Llewellyn (16 years her junior), Dempster asked Llewellyn to stop working in his garden and begin talking into his tape recorder.

"There isn't anything I don't know about Roddy," Dempster said with some weariness between interviews with European royalty attending a recent Courvoisier backgammon tournament in the Bahamas. "After a few months I'd written 50,000 words, and my agent suggested turning it into a book. 'About Roddy?' I asked. He said, 'No, you ninny, about Princess Margaret!'"

Dempster says her royal highness cooperated slightly, correcting information in a 17-page synopsis he provided her prior to publication.

"Throughout she kept saying, 'You will be kind to Roddy, won't you?'" Dempster recalls. "She went into a bit of a spin after it first came out, but we're back on speaking terms now."

Dempster earned some criticism for being a traitor to his class, or at least his wife's class: He is married to the daughter of the duke of Leeds, who, says Dempster airily, understands his book is "a commercial enterprise."

Dempster is not in awe of his subject.

"I once said, 'Surely you have some ambition?' 'No,' she said, 'actually I don't.' Now here was a woman who had everything, yet she couldn't have everything. Two dukes turned her down and when she wanted to marry Peter Townsend, her sister who ruled over everything she saw couldn't help her ... In a world of lessening standards, to have the feeling that something as permanent and incorruptible as (the British royal family) has someone in its group who has been corrupted like Princess Margaret, it gets fascinating. Everyone wants to see the downfall of others. It makes you feel better."