McMaster University has the best/most usable collection.
These publications from the war explain how the maps were made, how accurate they were, how they should be used (i.e. 1:100,000 scale maps were recommended for use by armored units), if they were available, plans to produce more/future availability, etc. For example, topo lines in Germany were interpreted from hatch marks on German maps from the 1930s so were good for approximation and strategic planning, but were not precise.
G.S.G.S Maps of France, Belgium and Holland
G.S.G.S. Maps of Italy, Sicily, Sardinia, and Corsica