Mayer, G. (1975). Friedrich Engels. Ullstein.
Lua i Stíl Chicago (17ú heag.)Mayer, Gustav. Friedrich Engels. Frankfurt am Main: Ullstein, 1975.
Lua MLA (8ú heag.)Mayer, Gustav. Friedrich Engels. Ullstein, 1975.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.