Modern Germany : an Encyclopedia of History, People, and Culture, 1871-1990 (1998) [Ref] DD14 .M64
Germany: a Reference Guide from the Renaissance to the Present (2006) [Ref] DD17 .B54
Encyclopedia of German Resistance to the Nazi Movement (1997) [Ref] DD256.5 .L51313
Who's Who in Nazi Germany (1995) [Ref] DD256.5 .W564
Nazi-Deutsch / Nazi German: an English Lexicon of the Language of the Third Reich (2002) [Ref] PF3680 .M48
In getting started, why would you need a reference book when you could just use "google" or head for Wikipedia?
The titles on this page are authoritative, informative, and concise. Of the three, authoritative counts the most.
Oxford Companion to World War II (1995) [Ref] D740 .O94
Routledge Atlas of the Second World War (2009) [Ref] G1038 .G5
Times Atlas of the Second World War (1989) [RefAtlas] G1038 .T6
Encyclopedia of the Holocaust (1990) [Ref] D804.3 .E53
The United States Holocaust Memorial Museum Encyclopedia of Camps and Ghettos, 1933-1945 (2009) [Ref] D805.A2 U55
The Yad Vashem Encyclopedia of the Ghettos During the Holocaust (2009) [Ref] D805.A2 Y33
Bearing Witness : a Resource Guide to Literature, Poetry, Art, Music, and Videos by Holocaust Victims and Survivors (2002) [Ref] D804.3 R67
Historical Atlas of the Holocaust (1995) [Ref] G1797.21.E29 H5