"I'LL TAKE THE DARK ONE," says Antonia Fraser, singling out the brooding figure at a nearby table. The restaurant is Étoile in London's Fitzrovia. The time is the mid-1970s, and "the dark one" is Harold Pinter, looking dashing over lunch with fellow thespians. So begins the affair between Pinter, Britain's leading playwright of the postwar period, and Fraser, the biographer who made her name with the 1969 best seller Mary Queen of Scots. He is a stormy creative titan, widely acknowledged as the heir to Samuel Beckett. She is a beautiful, fair-haired aristocrat who has carved out a place as a popular historian by writing the lives of royal women. In a milieu of scruffy hotel bars, opening nights, and family homes, they promptly recognize each other's glamour and fall in love, not just with each other, but also — and perhaps more importantly — with the idea of each other.

This is no springtime romance. The lovers are both in their forties, and both married. They come from different worlds: she from a privileged upbringing in North Oxford, he from a Jewish family in the East End. They cross paths now because they move in the same circles of literary prestige. Fraser's husband is an easygoing Tory MP with whom she has six children. Pinter's wife is actress Vivien Merchant, who originated what he called the "mysterious, sexy Pinter Woman" onstage in the late 1950s. No strangers to extramarital romance, the new lovers are surprised by the strength of their feelings. And so they set about dismantling their respective lives and forging one of the most enduring partnerships in British public life. Must You Go? chronicles the three decades they spent "famously married," writing, traveling, campaigning, loving, and quarrelling, until Pinter's Nobel Prize win in 2008, followed shortly after by his death from cancer at 78.

Fraser adds her book to a genre with a long history: the marriage memoir. When husbands attempt it, the results tend to come trailing clouds of scandal and betrayal. After Mary Wollstonecraft died in childbirth, her husband William Godwin published a record of her unconventional life. In Memoirs of the Author of A Vindication of the Rights of Woman, Godwin laid out her highs and lows, her love affairs and suicide attempts, to clear the record and establish her genius. But instead of enshrining his wife's reputation, Godwin's book had the effect of tarnishing it for nearly 150 years. Two centuries later, British poet laureate Ted Hughes reportedly went into hiding for nineteen days following the publication of Birthday Letters, the 1998 volume in which he finally broke the silence about his marriage to Sylvia Plath. The poems, assembled from across the years, were as contentious as could be expected: Marjorie Perloff called them "the unkindest cut of all."

If widowers' tell-alls are a risky (and sometimes risqué) business, then women's counterparts have more often been decorous catalogues of What the Great Man Ate for Breakfast. In the hands of Victorian widows, it was a genre of genuflection. Florence Hardy's two volumes of reminiscences, Early Life of Thomas Hardy and Later Years of Thomas Hardy, were ghostwritten by her husband. Even the spirited Katherine "Kitty" O'Shea does her duty, in her memoir of the Irish liberator Charles Stewart Parnell, delivering the expected record of meetings and decisions. But luckily, in a delicious later chapter called "Parnell as I Knew Him," she at las...