Here's the best opn-line discussion of the battle I've seen:

Retreat from Gettysburg: Lee, Logistics, and the Pennsylvania Campaign
By Kent Masterson Brown, Esq.

https://books.google.com/books?id=1L9RtUE9LhwC&pg=PA236&lpg=PA236&dq=Black+teamsters+at+Williamsport&source=bl&ots=rzNijZl7Cy&sig=Dycyqj4fWKXHFBOVvDiJG9PU-AE&hl=en&sa=X&ved=0ahUKEwih8KzA7-DbAhVJhq0KHYJDBvoQ6AEIazAJ#v=onepage&q=Black%20teamsters%20at%20Williamsport&f=false