Paris may have been bustling, but it was not necessarily a happy or safe place. One novel that shows the other side of the city during the war is Trapeze by Simon Mawer [F MAW, also AB/CD MAW]. It is the story of a young British woman who is trained as a spy and sent to France, and then to Paris.
The danger of Paris is a cancer within you, invisible, imponderable, and probably incurable.A great read! I recommend it highly.