Go to the site given, see the picture of the 1939 Poland map boundaries (Click on the underlined word, 'partitioned' to find the first multi-color map given covering 1939-1941).
1939: Poland ceases to exist once again after being partitioned between Nazi Germany and Soviet Russia at the outbreak of World War Two.
1945: Poland re-emerges on the map following the end of World War Two as the People’s Republic of Poland, a Soviet satellite state. As a result of extensive territorial changes, Poland moves several hundred kilometres to the west, losing its former eastern territories to the Soviet Union.
There are different World Atlases in area public and academic libraries which show boundaries...look in the Reference Sections under 910-912 or so in Dewey Decimal Classifications of public libraries. History in the academic Library of Congress Classification system should be in D (European History), G (Geography), and J (Political Science).