Started in 1937 in ChinaIf you consider it started before 1939 (indeed, there were some first signs in Spain...), then go to Hemingway's "For whom the bell tolls".
May be you can find "The silence of the sea" by Vercors, dealing with German occupation in France.
If you're interested in the other party point of view, let's try "'The kindly ones" by Jonathan Little( big book, will keep you busy for a while), or also "Death is my trade", by Robert Merle.
R