The End of the Affair by Graham Greene

Set in London during and just after World War II, the novel revolves around a love affair between Maurice Bendrix, a writer, and Sarah Miles, the wife of a civil servant. The story is narrated by Bendrix, who is obsessed with Sarah and hires a private investigator to follow her when he suspects she's having another affair. The novel explores themes of love, hate, and the existence of God, with Sarah's faith playing a significant role in the narrative.

The 275th greatest book of all time