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