This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Tempest: A Novel
When 007 Holly read the card Jackson made for Courtney, it is absoutely devastating, especially so because Courtney never got to read it.
Courtney's death. We know what's going to happen, but being there to watch it just drives the nail in deeper. The fact that she was too weak and tired to even care about the holes in Jackson's story, and the way her pain is conveyed is truly heart wrenching.
At the end of the book, when Jackson jumps back to the beginning of 2009 and watches Holly just walk out of the building and go on with her life, after his promise to Courtney.