Eric Foner’s Gateway to Freedom.
Actually, I don’t recommend this book. The New York Times does. I haven’t read it yet. But I will because (1) Eric Foner admitted me to the graduate program in history at Columbia (and I’ve forgiven him for losing my only copy of Joseph R. Holmes’ photograph), and (2) in the 1930s, Ardeanur Hart worked for a descendant of Sidney Howard Gay at the family’s Staten Island house, formerly an Underground Railroad station.