Dear Ms. Baker,
Thank you for contacting History Hub!
One avenue we suggest is contacting the Library of Congress. They have several historical address/telephone directories in their collection from Germany. For more information, please visit https://www.loc.gov/rr/european/tel.html. To contact a Reference Librarian from the European Division please visit https://www.loc.gov/rr/european/eurspecialists.html
We also suggest you contact the Family History Center in Landshut, Germany. While the web page is in German, there is an email address listed that you can use to send them inquiries. They should be able to tell you if address books for 1879 still exist and if they are available to view online.
Ancestry also has a database that may be useful as well: Germany and Surrounding Areas, Address Books, 1815-1974.
Best of luck in your research!