"Family history begins with missing persons," Alison Light writes in Common People. We wonder about those we've lost, and those we never knew, about the long skein that led to us, and to here, and to now. So we start exploring. Most of us, however, give up a few generations back. We run into a gap, get embarrassed by a ne'er-do-well, or simply find our ancestors are less glamorous than we'd hoped. That didn't stop Alison Light: in the...