I read a certain book several years ago that continues to haunt me. It is called Embers by the Hungarian author Sandor Marai.
It haunts me because it is really all about one brief moment when a decision is made, most likely based on love, that changes three peoples' lives forever. It is particularly rich because of the intersecting themes--friendship, romantic love, loyalty and the passage of time. It is a deep and moving book that anyone could relate to on some level.