The British BBC radio Channel 4 has a very worthwhile (IMO) programme called in In Our Time. I have downloaded many episodes for listening in the car or on a long plane journey. Anyway you may be interested in this one. It takes a while to cover basic issues (remember it is aimed at a UK audience). But I think it makes some interesting comments on 'why' the Address was so important and why Lincoln needed to make a major speech at that time. Make no mistake Lincoln was a canny politician and I'm sure that he viewed this as a critical time in his Presidency. :-
The Gettysburg Address