Solomon Northup’s story, which has been studied by historians for decades, now has a second life in American popular culture, thanks to director Steve McQueen’s extraordinary movie “12 Years a Slave.” The film — nominated for nine Oscars, including best picture and best director — brings Northup’s remarkable 1853 memoir to life with searing portrayals of torture and survival. It has revived curiosity about Northup’s life and renewed debate over how to depict the pain of the past and the present. Does McQueen’s movie go too far with violence?

An answer may be found in the diary of a Union soldier named John Burrud. Ten years after Northup was rescued from slavery in Louisiana, Burrud marched through the neighborhood where Northup had been held captive by the brutal cotton planter Edwin Epps. The soldier knew the story and recognized where he was. “Resumed our march 4 o Clock AM followed Bayou Beauf down,” he scribbled in his little leather-bound pocket diary on May 18, 1863. “I think this is the place that Solomon Northup operated.”

I stumbled across Burrud’s diary just before Christmas in the reading room of the gorgeous Huntington Library in San Marino, Calif., which is a long way from Avoyelles Parish, La., or Saratoga Springs, N.Y., where Northup lived. I had no idea when I opened it that Burrud’s diary would shed any light on Northup’s story and its legacies. In fact, I had been looking for something entirely different. History is full of surprises....