Edith to replace parents
The character of Edith is a symbolic portrayal of the emotional baggage that Stoner carries throughout life because of the low self-esteem instilled in him by his parents. Edith thinks very lowly of Stoner, and she doesn't understand him nor does she want to. That's the exact same relationship Stoner used to have with his agrarian parents. This symbolizes the cycle, that Stoner (in order to work through the troubles of his childhood) accidentally fell in love with someone who would abuse him in the same way.
The motif of abuse
When Stoner cheats on Edith, it follows a long, long road of abuse, neglect, mistreatment, and coldness. Edith is a stone-cold wife who doesn't give her husband one ounce of love, ever. She hates her daughter because the daughter loves Stoner, and she sabotages that relationship too. Stoner eventually has an affair, and Grace grows up (abused). The motif here is not only abuse, but a cycle of abuse where people who are abused tend to abuse others in the same ways that they were abused.
The ending at the beginning
Another symbolic representation of cycles (like the cycle of abuse, the cycle of neglect, etc.) is the cycle of time that the book builds by putting the ending of the story at the beginning of the book. This is a subtle reminder to the reader that the human life is mysterious and unexplainable, and that the mysteries of death are completely unknown to the human mind. The cycle here is that the character dies, but Grace has the same childhood that Stoner had (basically), and now Grace is the new "Stoner," as she goes through her own story toward peace. It is a life/death cycle (which is perhaps why the novel begins with his death).
The motif of literature
Literature itself appears in this book as a symbol for meaning and love. When Stoner was lonely and directionless, it was literature that saved him. He found that there was an infinite, loving family available to him in the form of the written word. That provoked him to literally find community in the literature department of his university. When he figures out the true meaning and value of his human life, he writes a single book, another sign of literature as meaning and love (because it indicates that he has come to love the strange, difficult, lonely meaning of his life).
Grace as a metaphor
Grace is literally made of her father's DNA, so she is a metaphor for his story continuing on past his death. It is a kind of reincarnation, and the first time the story happens to a man, but the second time, it happens to a daughter (an opposite, for balance). Up close, we see the tail end of Stoner's difficult life, but in the distance, we see the beginning of Grace's journey (of coping with her history of trauma and maternal abuse, and of being widowed by WWII). As a metaphor, she represents the universal nature of human struggle and pain, because she has the same pains that her parents have from their own childhoods.
