When I play wargames I regularly read books on the subject, view movies, and do some background research. For Dark Summer only the first few turns are well covered by the many publications on D-Day. One interesting source I came across recently is the Imperial War Museum’s website with several articles on D-Day and Normandy. The German Response to D-Day, What Happened After D-Day, and Tactics and the Cost of Victory in Normandy are but a few such articles that will give you more insight into this campaign.
Another book I read some time ago and that I much recommend is If You Survive (George Wilson) written from the perspective of a junior infantry officer that took part from the Saint-Lô breakthrough July 1944 to the assault on the Siegfriedlinie February 1945.