The Mark Twain quotation seems particularly pertinent to Zora Neale Hurston's Their Eyes Were Watching God.
In many ways the novel (an all of Hurston's work) is certainly an example of an ethnographic text. Hurston's background as a folklorist and anthropologist certainly establish her knowledge and experience in portraying the lives of emerging African American culture and its many manifestations. We also see the interactions between that emerging African American consciousness and the larger Euro-American culture that all too often oppresses it. Their Eyes is most certainly about race.
However, it is also about space--not only the importance of setting and physical location (the all-black town of Eatonville is a prime example) but also the space for personal freedom and the space for the quest for self-identity and self-understanding. We see this later idea best expressed in the recurring metaphor of the horizon. From the novel's opening paragraphs to its final message at the end, we witness Janie's quest to reach and understand her horizons, to understand the "lay of the land" and the space around her.