The Scarlet Letter by Nathaniel Hawthorne (1850):
After being abandoned by her husband, Hester Prynne has a secret affair with Reverend Arthur Dimmesdale and bears his child. Shunned by the community and forced to wear a scarlet letter "A" (for "adulterer") across her chest, she wrestles with her feelings of guilt.
Madame Bovary by Gustave Flaubert (1856):
Emma Bovary, the wife of Dr. Charles Bovary - a middle class and rather unremarkable man - has numerous extramarital affairs in hopes of finding an affection, social status, and wealth her marriage can't provide.
Anna Karenina by Leo Tolstoy (1877): Anna Karenina is torn between her desire to be an independent woman - free to find true love with Alexei Kirillovich Vronsky - and her strong sense of marital and familial obligation.
Lady Chatterley's Lover by D.H. Lawrence (1928): When Lady Chatterley's husband returns from war paralyzed from the waist down, she has a passionate affair with the gamekeeper, Oliver Mellors. In a moment of sexual awakening, she realizes that physical pleasure is an important component of a satisfied life.
Dr. Zhivago by Boris Pasternak (1957): Poet-physician Yuri Zhivago's life is greatly disturbed when, in the process of moving his family to the Ural Mountains, he gets involved in the Russian Revolution and an affair with Lara, the beautiful wife of a revolutionary soldier.
The Photograph by Penelope Lively (2002): After finding an old photograph of his wife Kath holding hands with another man, Glyn Peters becomes obsessed with her act of infidelity and recklessly delves into the past to find out what happened.