Eric Foner has won a place in the front rank of American historians with books that seem to vacuum up all available sources to produce bold new interpretations of the country’s reckoning with the big questions of slavery and freedom.

But his latest grew from a modest beginning: a tip from his dog-walker.

In 2007, Madeleine Lewis, an undergraduate history major who helped with the family cocker spaniel, had been looking at the papers of a little-known 19th-century abolitionist editor named Sydney Howard Gay, held at Columbia University, when she came upon a small notebook labeled Record of Fugitives.

She mentioned it to Mr. Foner, who was busy writing “The Fiery Trial,” his Pulitzer Prize-winning study of Abraham Lincoln’s shifting views of slavery. A few months later, he got around to looking at the notebook, which contained detailed records of Gay’s efforts to help more than 200 runaway slaves passing through New York City.

“I was amazed,” Mr. Foner said recently during an interview in his office at Columbia, where he has taught since 1982. “I had never heard of this document, or seen it cited.”

He added: “Normally, I start with a historical question and then go looking for documents that might help me answer it. This was the first time it happened the other way around.”