Edith Wharton was the first woman ever awarded the Pulitzer Prize, and she won it for this novel in 1921. Here's a plot summary from Wikipedia:
Newland Archer, gentleman lawyer and heir to one of New York City's best families, is happily anticipating a highly desirable marriage to the sheltered and beautiful May Welland. Yet he finds reason to doubt his choice of bride after the appearance of Countess Ellen Olenska, May's exotic, beautiful thirty-year-old cousin, who has been living in Europe. Ellen has returned to New York after scandalously separating herself (per rumor) from a bad marriage to a Polish count. At first, Ellen's arrival and its potential taint to his bride-to-be's family disturbs him, but he becomes intrigued by the worldly Ellen who flouts New York society's fastidious rules. As Newland's admiration for the countess grows, so does his doubt about marrying May, a perfect product of Old New York society; his match with May no longer seems the ideal fate he had imagined.
The summary on Wikipedia also has a list of the few major and the many minor characters so you might want to keep the list handy.