In 1984, the landscape historian Mac Griswold was rowing along a Long Island creek when she came upon Sylvester Manor, a stately mansion guarded by hulking boxwoods. When Griswold went inside, she encountered a house full of revelations, including a letter from Thomas Jefferson and--most remarkable and disturbing--what the aged owner, Andrew Fiske, casually called the "slave staircase."

This staircase would reveal the extensive but little-known story of Northern slavery, and in 1997 Griswold returned with a team of archaeologists, uncovering a landscape filled with stories. Based on...