The Mission of the Center for Military History (CMH): "To accurately collect, preserve, interpret, and express the Army's history and material culture to more broadly educate and develop our force, the military profession, and the nation." To that end, CMH publications include many primary source materials that can be excellent sources for research on military operations. Some examples of primary and secondary sources include the items listed below.
The Combined Arms Research Library (CARL) Digital Library contains a great deal of information on Operation Cobra. Resources in this library include both secondary and primary sources. Below are examples of the type of information you can get online. Follow this link to search for your area of interest: CARL Digital Library.