The reference tools shown here will include both paper and electronic resources, as both can be very important. These specialized dictionaries and encyclopedias are great for providing a good overview of a topic (written by experts in the field), as well as the best bibliography for further exploration. They are often the best gateway into any topic you do not already know well. The nomenclature is not standard, but I have chosen to list under Dictionaries, single volume works, and works with relatively short entries (like language dictionaries, no matter how many volumes). Encyclopedias are multi-volume works with longer entries.
This series from Garland/Routledge is very good for specific countries/regions. They are currently only in paper, but there are plans to create a digital product incorporating them.