A Pulitzer Prize-winning book on a brutal aspect of U.S. history has reignited debate on the country's racial past just as the country's first black president is seen as evidence of racial progress.

"Slavery By Another Name" recounts the little-known story of how in the decades after President Abraham Lincoln's 1863 Emancipation Proclamation to free slaves, hundreds of thousands of black Americans were re-enslaved as convict laborers.

Author Douglas Blackmon said on Tuesday the story was "absolutely essential" to understanding why a U.S. racial divide still exists and why the country's black minority lags behind the rest of the population in terms of economic and social health.