This standard reference is a two-volume set found in most genealogy libraries, and can be invaluable when trying to figure out where records are kept for various German cities, towns, and villages.
It can be daunting to some degree because it is in German (Gothic script), but a little perseverance and careful reading of the early pages that provide abbreviations and guides on how to use the book, will pay off.
It includes all areas that were part of the German Empire before WWI.
Meyers is also available on Ancestry.com for free (you don't need a subscription). The link below will take you to the search page, but scroll down and read the database description before you do any searching.
Meyers Gazetteer of the German Empire:
'via Blog this'