For obvious reasons, the story of Michelle Obama’s long-dead relatives has also attracted attention. Rachel L. Swarns summed up the facts in a 2009 New York Times article that led to her new book, “American Tapestry”: “[A] union, consummated some two years before the Civil War, represents the origins of a family line that would extend from rural Georgia to Birmingham Alabama to Chicago and finally to the White House. Melvinia Shields, the enslaved and illiterate young girl, and the unknown white man who impregnated her are the great-great-great-grandparents of Michelle Obama, the first lady.”

In “American Tapestry,” Swarns traces both sides of Mrs. Obama’s family back to the 1840s, building carefully to the story of Melvinia Shields. Swarns’s research is extensive and meticulous — one feels the hours that she spent poring over old documents and talking with genealogists and historians. Her passion for the story is clear and striking.

What works against her, and against the full success of the book, is the sketchiness of the history that slavery created and enforced and the rareness of literacy among slaves and their immediate descendants. Very few letters, journals or notes have survived — all the written ephemera so crucial to the historian in discovering the mindset of his or her subjects. Further, many of those in the generations immediately after slavery maintained a staunch silence about the experience, as though to blot out the horror.

Because of that silence, African Americans tend to lack stories from “the old country.” Without these anecdotes and handed-down stories, many of those whom Swarns interviewed have slender and unenlightening memories, as in this quote from Nomenee Robinson, remembering his grandmother Phoebe Robinson, Mrs. Obama’s great-great grandmother on her father’s side: “She was the keeper of wonderful recipes. . . . Rhubarb pie. I’ve never had it since. She made dandelion greens, dandelion soup. That I remember very well.”

We can’t be privy to what was on the minds of Mrs. Obama’s long-dead ancestors as they moved through their challenging lives. Swarns’s effort to create vivid characters and a strong narrative involves a great deal of hypothesizing about what a person might have been thinking or feeling, as here, when she describes Melvinia’s home in Georgia when she was 8: “Blacks and whites often played together as children, and Melvinia may have found some solace in the silly games and childish diversions. She could have closed her eyes and, for a moment, felt just like any other little girl, lost in the joy of play.” Swarns is striving here to imagine the inner lives of the long-dead, but the “could have/might have” formulation, which appears over and over throughout the book, does not create a convincing picture of those inner lives.