What was it like to be a slave?  It was a question that went unasked for more than 70 years, until, in 1937, the Federal Government commissioned investigators with notebooks and cameras to seek out people who had been born into slavery.  Richard Cahan co-edited the new book River of Blood: American Slavery from the People Who Lived It which culls some of the best photos and most poignant stores of Americans who once were slaves.  Another example of how much can learn from once-hidden history.