The Wikipedia summary:
Forever Amber tells the story of orphaned Amber St. Clare, who makes her way up through the ranks of 17th century English society by sleeping with and/or marrying successively richer and more important men, while keeping her love for the one man she could never have. The novel includes portrayals of Restoration fashion, politics, and public disasters, including the plague and the Great Fire of London.
While many reviewers "praised the story for its relevance, comparing Amber's fortitude during the plague and fire to that of the women who held hearth and home together through the blitzes of World War II", others condemned it for its blatant sexual references. Fourteen U.S. states banned the book as pornography. The first was Massachusetts, whose attorney general cited 70 references to sexual intercourse, 39 illegitimate pregnancies, 7 abortions, and "10 descriptions of women undressing in front of men" as reasons for banning the novel. Winsor denied that her book was particularly daring, and said that she had no interest in explicit scenes. "I wrote only two sexy passages," she remarked, "and my publishers took both of them out. They put in ellipsis instead. In those days, you know, you could solve everything with an ellipsis."
Despite its banning, Forever Amber was the best selling US novel of the 1940s. It sold over 100,000 copies in its first week of release, and went on to sell over three million copies.
And from a 2002 re-appraisal by Elaine Showalter:
The novel came out in England in 1945. While English women were weeping over Celia Johnson's stoic portrayal of sexual renunciation in Brief Encounter they were also bonding over the bawdy, upwardly-mobile Amber St Clare. Forever Amber was published at a time of social upheaval in Britain, the beginnings of the welfare state and the erosion of an ethic of social and marital deference. Divorce petitions skyrocketed during the war, rising from 9,970 in 1938 to 24,857 in 1945. Moreover, Winsor's readers, the majority of them women, identified with Amber's calamitous life and admired her fortitude in times of hardship. The great fire of London would have seemed familiar to those who had had lived through the blitz. The random nature of plague would ring true for those who had lived with the constant fear of buzzbombs and V2 rockets.
In its day Forever Amber was a standard reference in popular culture, such as in this scene from The Jack Benny Program:
Old Man: I just finished reading a sad book.
Jack Benny: What was it?
Old Man: Forever Amber
Jack Benny: Forever Amber? That’s not a sad book.
Old Man: It is when you’re my age, Bub!
And for another racy book see Robert McHenry's post on the Britannica Blog on the 50th anniversary of the banning of Lady Chatterley's Lover.