Amy as a symbol for longing
Amy is a femme fatale in a way, because she represents the woman that Dorrigo longs for but cannot have, and since she is forbidden to him, having already married Dorrigo's uncle. The worst part is that Dorrigo knows that she would probably leave her uncle for Dorrigo, but Dorrigo doesn't find out she's still alive until it's too late. More broadly, she represents Dorrigo's desire for life to be different, a strong desire from a man whose life is so genuinely difficult.
War Camp Prison as a metaphor
Before exploring this further, it's important to remember that the events of this novel are historical and accurate, even though it is fiction, but nevertheless, it is clearly structured in a meaningful way. The reader can see that the Japanese POW camp represents something more than itself. In a way, it also represents the mental prison of isolation and post-trauma that Dorrigo suffers after his imprisonment. In other words, the reader can see that even when the war is over, Dorrigo's mental and emotional suffering continues.
Ella as a symbol
There is an Idealist/Realist dilemma in the book, symbolized by Ella. Because Dorrigo can never get over the trauma and shame of his life, he is unable to appreciate the real opportunities he has at a happy, healthy life, and he costs his family a great deal of love and joy. Dorrigo cannot fall in love with Emma, because he is already in love with Amy, who represents the way he wanted life to be. Emma represents the way life is in reality, without regard for Dorrigo's wishes. Dorrigo never quite reconciles himself to reality.
The symbology of the title
The title of this novel is The Narrow Road to the Deep North, words taken from an important Japanese poet, Matsuo Basho. Basho's writing is about detachment from the earth's woes by way of meditation. In other words the title is a way of offering a hopeful interpretation of the novel. If perhaps, the detachment of Dorrigo (who represents the author's father) is actually a reflection of his life-long attempt to meditate his way into peace after serious PTSD and disappointment in life, then maybe his dark, horrifying story actually has private victories in it; perhaps Dorrigo privately enjoyed emotional and spiritual enlightenment in his meditation on trauma.
The desire for love motif
Dorrigo ends up feeling obligated by Ella early in their story because Ella is desperate for love, so she takes his words to mean more than they do, and by way of misunderstanding, the two are engaged. Also, Dorrigo thirsts for love, but not from Ella; from Amy, his uncle's hot wife. In the end, Dorrigo's own children thirst for his love, but they can never quite tell if Dorrigo doesn't love them because of his post-traumatic stress or because of some unsettled business with Ella. The reader knows that it's both.
