The book isn't exactly overwhelmed with symbolism, but one possible symbol you might like to consider is Nick's journal. Nick's forced to write the journal by the judge as part of the conditions of the restraining order taken out against him by Caitlyn. The judge tells Nick that he must write 500 words in the journal each week, detailing everything that's happened between himself and Caitlyn, from the moment they first met to the day of the court hearing. It's hoped that Nick will find the experience educational. He certainly has a lot to learn given his arrogant, disrespectful demeanor in court.
The journal could be said to symbolize the personal voyage of self-discovery that Nick undergoes throughout the course of the book. In writing about his tumultuous relationship with Caitlyn, he's forced to come to terms with his abusive behavior towards her. Not only that, but he's forced to confront the source of all the pent-up rage inside him, the years of physical abuse to which he himself has been subjected by his father.