If you seen George Clooney's movie based on the book, The Monuments Men , you may be interested in reading more about the international efforts to rescue art from the hands of the Nazis.
The Rape of Europa: The Fate of Europe’s Treasures in the Third Reich and the Second World War 
Rescuing Da Vinci: Hitler and the Nazis Stole Europe’s Great Art – America and Her Allies Recovered It 
Nazi plunder : great treasure stories of World War II 
The lost museum : the Nazi conspiracy to steal the world's greatest works of art