In Mama Day, what is the relationship between the three prefatory documents (bill of sale, map, and family tree) and the narrative proper?
In the preface, the bill of sale explains how Sapphira Wade came to be Bascombe Wade's slave. The bill of sale also gives credence to Sapphira's legendary status as a "conjure woman." It states that Sapphira was of a "bilious nature" and often resisted commands to perform field or domestic labor. She was also known to have presided over births as a midwife or nurse, often "not without extreme mischief" during the performance of her duties.
Last, but not least, the bill of sale notes that Sapphira dabbled in witchcraft. So, the bill of sale reinforces the narrative of Sapphira as a sorceress of sorts:
She could walk through a lightning storm without being touched; grab a bolt of lightning in the palm of her hand; use the heat of lightning to start the kindling going under her medicine pot...she turned the moon into salve, the stars into a swaddling cloth, and healed the wounds of every creature walking up on two or down on four.
The narrative certainly credits Sapphira's "conjuring" powers for inspiring Bascombe to bequeath Willow Springs to his slaves. It is also likely that Sapphira deceived Bascombe as to her true intentions when she married him.
Meanwhile, the family tree delineates how Cocoa (the heroine of the novel) descended from Sapphira Wade. The family tree also reinforces the narrative that Sapphira birthed seven sons; the tree names them as Elijah and Elisha (twin boys), Joel, Daniel, Joshua, Amos, and Jonah Day. The family tree also shows that the youngest of Sapphira's sons, John Paul, was father to Miranda (Mama Day).
Most importantly, Mama Day is linked to Cocoa because the latter is one of Mama Day's great-nieces. Both Mama Day and Cocoa are central characters in the narrative.
Meanwhile, the map reinforces the narrative of Willow Springs as a unique parcel of land, neither part of South Carolina nor Georgia. Willow Springs is shown as an island on the map. On the island, the map notes the positions of Mama Day's trailer, Ruby's house, Bernice and Ambush's house, and Abigail's house. In the narrative, Mama Day is central to the lives of Bernice, Ruby, and Cocoa. Meanwhile, Mama Day and Abigail raised Cocoa after the death of Cocoa's mother. On the map, Abigail's house is just across the street from Mama Day's trailer.
The narrative of Mama Day is in brief Cocoa's quest for identity; her attempt to blend her secret past, bound up in magical Willow Springs, and her future, bound up in New York City. Cocoa's quest takes the reader to Willow Springs, which is Cocoa's home, and the legend of Sapphire, whose name is never uttered on the island of Willow Springs. The prefatory documents begin and corroborate the story of Sapphire, legendary for her association with and influence over Bascombe Wade, who prompted by Sapphire's urging and after "a thousand days" deeded all the land of Willow Springs to his slaves.
The bill of sale authenticates Sapphire's existence, temperament, talents and status as a slave. The map gives authenticity and structure to Willow Springs, which doesn't exist on any other map as neither South Carolina nor Georgia--the borders of which Willow Springs is said to straddle--want to claim it or have anything to do with it. The family tree shows the seven sons born to Sapphire fathered by Bascombe Wade in "a thousand days" and shows the direct line of descent from Sapphire to Cocoa, the heroine of the story.