See Ted Alexander, "A Regular Slave Hunt," North & South Magazine, Vol. 4, No. 7. Sept. 2001.
Also Peter C. Vermilya, "The Effect of the Confederate Invasion of Pennsylvania On Gettysburg's African-American Community," Gettysburg Magazine, online at http://www.gdg.org/Gettysburg%20Magazine/gburgafrican.html (additional sources are in the bibliography)