1 Nocturnal Animals has two endings
The fictious book ends with Tony shooting himself by accident and the movie (real world) ends with Susan waiting for Edward as he never shows up.
2 Susan didn't believe in Edward
Susan didn't believe in Edward or his work because she is a 'realist' and he is a sensitive man. He writes about himself and she doesn't like his work.
3 Susan hurt Edward greatly
Susan hurt Edward greatly by getting an abortion. She thought he wouldn't know but he was there, watching her when she left.
4 The book presents reality
The book Edward wrote presents reality. The rape of Tony's wife and daughter shows how Edward lost Susan.
5 We see everything from Susan's point-of-view
Susan's point-of-view is all that the audience has access to. She makes Edward present Tony, yet she does not present the wife in her mind. She feels guilt throughout the movie because of the book. Even the flashbacks are from her side not Edward's.
6 The book presents Susan and Edward
While Susan sees Edward as Tony, Edward has also made Bobby to be his subconscious, his powerful side. At the same time, Ray is the dark side of Susan who has killed Laura and took over.
7 Ray shows Susan's nature
Ray shows the dark side of Susan; the one who took away Tony's (Edward's) child and the one who took away Laura (Edward's preferred side of Susan). Edward shows her that but she doesn't understand it at first. He portrayed who Susan became.
8 Bobby presents Edward's subconscious
Bobby presents Edward's subconscious; the voice in the back of his head telling him to keep going. When Tony dies, it is Bobby who survives even though we don't know that. It is through Bobby that he shows Susan that he is no longer weak.
9 The book is half of Edward's revenge
The book is Edward's revenge on Susan. It is what keeps her up at night and what makes her crave the freedom she had with him; the true love that she had with him.
10 He proved her wrong
The second part of his revenge is giving her hope and telling her that he would meet her when he really doesn't. He keeps her hanging to show her how much she is unhappy and how successful he had become. He gets his revenge fully when she realizes that he won't show up.