the difference between a memoir and a novel is in a million little pieces

Augustine's book has weight because he uses his own experience to explain his philosophy and give his it a frame of reference. Were it a work of fiction the philosophy gained through years of thought and experience could be dismissed more easily.

Similarly were Ellison's novel a true story readers wold be less likely to read deeper into it. The factual nature of an autobiography makes the use of metaphor and foreshadowing seem contrived. In works of fiction such literary tools is a mark of good literature and forces readers to look deeper than the surface for further allusions and hidden messages and themes.