Early in the movie, Erica (Jennifer Connelly) is reading Joan Didion's Blue Nights, which is about the death of Didion's daughter. At that point in the movie, Erica has completely lost Sam (Lily Collins), her daughter.
Rusty's story that Samantha sends to Stephen King is the repeated writings of the lyrics of The Beatles song "I've Just Seen a Face". In the movie, Rusty says that this song plays on his head every time he sees Kate. Also the story starts with Rusty's opening lines: 'Looking at her hurts'.
The trivia items below may give away important plot points.
When Sam and Bill are sitting down outside, Bill asks, "How's Lou?," and Sam responds, "Who's Lou?," implying that she doesn't want to see him anymore. However, at that moment she is reading an Ed Brubaker comic book, which she had earlier mocked Lou for saying was one of his favorite writers.
The book Rusty (Nat Wolff) gives Kate (Liana Liberato) before she gives him her "present" is a 25th anniversary edition of the book "It" by Stephen King, who in the movie Rusty is an avid fan of; there were only 3552 copies of the 25th anniversary edition of "It" ever put in to print. About 2750 unsigned copies, 750 signed copies, and 52 special signed and lettered copies. While it is later mentioned that Bill (Greg Kinnear) could get a signed copy of his ex-wife Erica's, (Jennifer Connelly), favorite book through his writers connections, this probably explains how Rusty would be able to get such a rare book, while at the time of it's release the book would go for it's retail value, it is now valued at at least $350 (2014).