I think this movie is actually about revenge.
The whole movie is based on two "parallel" stories. The events in real life are very similar to those of the book.
One is the relationship between Susan (Amy Adams) and Edward (Jake Gyllenhaal) a.k.a. real life where she reads the book. In this (lets say) universe she cheats on him and breaks his heart. At that point she is also pregnant with Edward's child (which we see at some point in the movie when Susan calls her). So basically, he loses them both.
The other story is where Tony's (also Jake Gyllenhaal) wife and daughter are killed in the night they where driving through nowhere.
Do you see how these stories are now similar? In both, Edward/Tony loses his wife and daughter.
Now in the book he seeks revenge for what happened to his loved ones and actually gets it. One guy is killed by Bobby (Michael Shannon) and the other Tony kills himself. On the other hand, in real life he gets his revenge by not showing up at the restaurant. He knows that the book would blow Susan's mind and bring back the feelings she had for him in the past. She also told him at some point that she doesn't think that he should write about himself in his books. But that's exactly what he did and he succeeded.
So in the end I think this is exactly what she realizes.
P.S. Another huge clue that made me come up with this theory is when Susan is at her art gallery. She pauses to look at the huge painting saying 'Revenge' and saying that she couldn't remember where did it come from.