Encyclopedias provide topic summaries, context, and bibliographies for further reading. Single subject encyclopedias offer much more depth in treating specific topics than can be found in other reference sources.
Yad Vashem Encyclopedia of the Ghettos During the Holocaust
Holocaust Literature: An Encyclopedia of Writers and Their Work
Encyclopedia of Holocaust Literature
The Holocaust and World War II Almanac
The New Standard Jewish Encyclopedia
United States Holocaust Memorial Museum Encyclopedia of Camps and Ghettos, 1933-1945
Rail entrance to Birkenau Concentration Camp. From http://www.jewishvirtuallibrary.org/jsource/Holocaust/birkrail.html