I had seen the movie based on the novel a long time ago The End of the Affair (1999) and I quite liked the film even though I found its ending a bit gloomy at the time.
The book evoked even stronger emotions for me. I'm not even sure whether to call this book just a run of the mill love story.
The second half is almost religious and spiritual. I found the novel quite nihilistic and religious at the same time.
Most of all, I like the brutal honesty of the protagonist who fights till the end to keep his nihilistic world view. I'm not sure whether it is nihilistic - at least he believed in love, so there's something!